The Collaborative International Dictionary of English v.0.48:

canonic \ca*non"ic\ (k[.a]*n[o^]n"[i^]k), canonical
\ca*non"ic*al\ (k[.a]*n[o^]n"[i^]*kal), a. [L. canonicus, LL.
   canonicalis, fr. L. canon: cf. F. canonique. See canon.]
   Of or pertaining to a canon; established by, or according to,
   a canon or canons. "The oath of canonical obedience."
   --Hallam.
   [1913 Webster]

   2. Appearing in a Biblical canon; as, a canonical book of the
      Christian New Testament.
      [PJC]

   3. Accepted as authoritative; recognized.
      [PJC]

   4. (Math.) In its standard form, usually also the simplest
      form; -- of an equation or coordinate.
      [PJC]

   5. (Linguistics) Reduced to the simplest and most significant
      form possible without loss of generality; as, a canonical
      syllable pattern. Opposite of nonstandard.

   Syn: standard. [WordNet 1.5]

   6. Pertaining to or resembling a musical canon.
      [PJC]

   Canonical books, or Canonical Scriptures, those books
      which are declared by the canons of the church to be of
      divine inspiration; -- called collectively the canon.
      The Roman Catholic Church holds as canonical several books
      which Protestants reject as apocryphal.

   Canonical epistles, an appellation given to the epistles
      called also general or catholic. See Catholic epistles,
      under Canholic.

   Canonical form (Math.), the simples or most symmetrical
      form to which all functions of the same class can be
      reduced without lose of generality.

   Canonical hours, certain stated times of the day, fixed by
      ecclesiastical laws, and appropriated to the offices of
      prayer and devotion; also, certain portions of the
      Breviary, to be used at stated hours of the day. In
      England, this name is also given to the hours from 8 a. m.
      to 3 p. m. (formerly 8 a. m. to 12 m.) before and after
      which marriage can not be legally performed in any parish
      church.

   Canonical letters, letters of several kinds, formerly given
      by a bishop to traveling clergymen or laymen, to show that
      they were entitled to receive the communion, and to
      distinguish them from heretics.

   Canonical life, the method or rule of living prescribed by
      the ancient clergy who lived in community; a course of
      living prescribed for the clergy, less rigid than the
      monastic, and more restrained that the secular.

   Canonical obedience, submission to the canons of a church,
      especially the submission of the inferior clergy to their
      bishops, and of other religious orders to their superiors.


   Canonical punishments, such as the church may inflict, as
      excommunication, degradation, penance, etc.

   Canonical sins (Anc. Church.), those for which capital
      punishment or public penance decreed by the canon was
      inflicted, as idolatry, murder, adultery, heresy.
      [1913 Webster]

WordNet (r) 3.0 (2006):

canonical
    adj 1: appearing in a biblical canon; "a canonical book of the
           Christian New Testament" [syn: canonic, canonical]
    2: of or relating to or required by canon law [syn: canonic,
       canonical]
    3: reduced to the simplest and most significant form possible
       without loss of generality; "a basic story line"; "a
       canonical syllable pattern" [syn: basic, canonic,
       canonical]
    4: conforming to orthodox or recognized rules; "the drinking of
       cocktails was as canonical a rite as the mixing"- Sinclair
       Lewis [syn: canonic, canonical, sanctioned]

Moby Thesaurus II by Grady Ward, 1.0:

89 Moby Thesaurus words for "canonical":
   Biblical, Christian, Gospel, Mariological, Mosaic, New-Testament,
   Old-Testament, abbatial, abbatical, accepted, apocalyptic,
   apostolic, approved, archiepiscopal, authentic, authoritative,
   binding, canonic, capitular, capitulary, churchly, clerical,
   confessional, conventional, correct, creedal, customary, dictated,
   didactic, divine, doctrinal, doctrinary, dogmatic, ecclesiastic,
   episcopal, episcopalian, evangelic, evangelical, evangelistic,
   faithful, firm, formulary, gospel, hard and fast, inspired,
   instructive, literal, mandatory, ministerial, of the faith,
   official, orthodox, orthodoxical, pastoral, physicotheological,
   preceptive, prelatial, prelatic, prescribed, prescript,
   prescriptive, priest-ridden, priestish, priestly, proper,
   prophetic, rabbinic, received, regulation, religious, revealed,
   revelational, right, rubric, sacerdotal, sanctioned, scriptural,
   sound, standard, statutory, textual, textuary, theological,
   theopneustic, traditional, traditionalistic, true, true-blue,
   ultramontane



The Jargon File (version 4.4.7, 29 Dec 2003):

canonical
 adj.

    [very common; historically, ?according to religious law?] The usual or
    standard state or manner of something. This word has a somewhat more
    technical meaning in mathematics. Two formulas such as 9 + x and x + 9 are
    said to be equivalent because they mean the same thing, but the second one
    is in canonical form because it is written in the usual way, with the
    highest power of x first. Usually there are fixed rules you can use to
    decide whether something is in canonical form. The jargon meaning, a
    relaxation of the technical meaning, acquired its present loading in
    computer-science culture largely through its prominence in Alonzo Church's
    work in computation theory and mathematical logic (see Knights of the
    Lambda Calculus). Compare vanilla.

    Non-technical academics do not use the adjective ?canonical? in any of the
    senses defined above with any regularity; they do however use the nouns
    canon and canonicity (not **canonicalness or **canonicality). The canon of
    a given author is the complete body of authentic works by that author (this
    usage is familiar to Sherlock Holmes fans as well as to literary scholars).
    ?The canon? is the body of works in a given field (e.g., works of
    literature, or of art, or of music) deemed worthwhile for students to study
    and for scholars to investigate.

    The word ?canon? has an interesting history. It derives ultimately from the
    Greek ????? (akin to the English ?cane?) referring to a reed. Reeds were
    used for measurement, and in Latin and later Greek the word ?canon? meant a
    rule or a standard. The establishment of a canon of scriptures within
    Christianity was meant to define a standard or a rule for the religion. The
    above non-techspeak academic usages stem from this instance of a defined
    and accepted body of work. Alongside this usage was the promulgation of
    ?canons? (?rules?) for the government of the Catholic Church. The techspeak
    usages (?according to religious law?) derive from this use of the Latin
    ?canon?.

    Hackers invest this term with a playfulness that makes an ironic contrast
    with its historical meaning. A true story: One Bob Sjoberg, new at the MIT
    AI Lab, expressed some annoyance at the incessant use of jargon. Over his
    loud objections, GLS and RMS made a point of using as much of it as
    possible in his presence, and eventually it began to sink in. Finally, in
    one conversation, he used the word canonical in jargon-like fashion without
    thinking. Steele: ?Aha! We've finally got you talking jargon too!?
    Stallman: ?What did he say?? Steele: ?Bob just used ?canonical? in the
    canonical way.?

    Of course, canonicality depends on context, but it is implicitly defined as
    the way hackers normally expect things to be. Thus, a hacker may claim with
    a straight face that ?according to religious law? is not the canonical
    meaning of canonical.


The Free On-line Dictionary of Computing (30 December 2018):

canonical

   (Historically, "according to religious law")

   1.  A standard way of writing a formula.  Two
   formulas such as 9 + x and x + 9 are said to be equivalent
   because they mean the same thing, but the second one is in
   "canonical form" because it is written in the usual way, with
   the highest power of x first.  Usually there are fixed rules
   you can use to decide whether something is in canonical form.
   Things in canonical form are easier to compare.

   2.  The usual or standard state or manner of
   something.  The term acquired this meaning in computer-science
   culture largely through its prominence in Alonzo Church's
   work in computation theory and mathematical logic (see
   Knights of the Lambda-Calculus).

   Compare vanilla.

   This word has an interesting history.  Non-technical academics
   do not use the adjective "canonical" in any of the senses
   defined above with any regularity; they do however use the
   nouns "canon" and "canonicity" (not "canonicalness"* or
   "canonicality"*). The "canon" of a given author is the
   complete body of authentic works by that author (this usage is
   familiar to Sherlock Holmes fans as well as to literary
   scholars).  "The canon" is the body of works in a given field
   (e.g. works of literature, or of art, or of music) deemed
   worthwhile for students to study and for scholars to
   investigate.

   The word "canon" derives ultimately from the Greek "kanon"
   (akin to the English "cane") referring to a reed.  Reeds were
   used for measurement, and in Latin and later Greek the word
   "canon" meant a rule or a standard.  The establishment of a
   canon of scriptures within Christianity was meant to define a
   standard or a rule for the religion.  The above non-technical
   academic usages stem from this instance of a defined and
   accepted body of work.  Alongside this usage was the
   promulgation of "canons" ("rules") for the government of the
   Catholic Church.  The usages relating to religious law derive
   from this use of the Latin "canon".  It may also be related to
   arabic "qanun" (law).

   Hackers invest this term with a playfulness that makes an
   ironic contrast with its historical meaning.  A true story:
   One Bob Sjoberg, new at the MIT AI Lab, expressed some
   annoyance at the incessant use of jargon.  Over his loud
   objections, GLS and RMS made a point of using as much of
   it as possible in his presence, and eventually it began to
   sink in.  Finally, in one conversation, he used the word
   "canonical" in jargon-like fashion without thinking.  Steele:
   "Aha!  We've finally got you talking jargon too!"  Stallman:
   "What did he say?"  Steele: "Bob just used "canonical" in the
   canonical way."

   Of course, canonicality depends on context, but it is
   implicitly defined as the way *hackers* normally expect things
   to be.  Thus, a hacker may claim with a straight face that
   "according to religious law" is *not* the canonical meaning of
   "canonical".

   (2002-02-06)