The Collaborative International Dictionary of English v.0.48:

Teacher \Teach"er\, n.
   1. One who teaches or instructs; one whose business or
      occupation is to instruct others; an instructor; a tutor.
      [1913 Webster]

   2. One who instructs others in religion; a preacher; a
      minister of the gospel; sometimes, one who preaches
      without regular ordination.
      [1913 Webster]

            The teachers in all the churches assembled. --Sir W.
                                                  Raleigh.
      [1913 Webster]

WordNet (r) 3.0 (2006):

teacher
    n 1: a person whose occupation is teaching [syn: teacher,
         instructor]
    2: a personified abstraction that teaches; "books were his
       teachers"; "experience is a demanding teacher"

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "teacher":
   Aaronic priesthood, Dutch uncle, Melchizedek priesthood, Polonius,
   Seventy, admonisher, adviser, advisor, apostle, backseat driver,
   bishop, boss, buttinsky, bwana, chef, chief, church dignitary,
   cicerone, coach, confidant, consultant, counsel, counselor, deacon,
   docent, doctor, dominie, don, ecclesiarch, educationist, educator,
   elder, employer, expert, fellow, forerunner, goodman, guide, guru,
   high priest, husband, instructor, kibitzer, lecturer, liege,
   liege lord, lord, lord paramount, master, meddler, mentor,
   mistress, monitor, nestor, orienter, overlord, padrone, paramount,
   paterfamilias, pathfinder, patriarch, patron, preceptist,
   preparationist, preparator, preparer, priest, professor, rabbi,
   sahib, schoolmaster, schoolmistress, schoolteacher, seigneur,
   seignior, starets, trailblazer, trainer, tutor