The Collaborative International Dictionary of English v.0.48:

Formula \For"mu*la\, n.; pl. E. Formulas, L. Formul[ae].
   [L., dim. of forma form, model. SeeForm, n.]
   1. A prescribed or set form; an established rule; a fixed or
      conventional method in which anything is to be done,
      arranged, or said.
      [1913 Webster]

   2. (Eccl.) A written confession of faith; a formal statement
      of foctrines.
      [1913 Webster]

   3. (Math.) A rule or principle expressed in algebraic
      language; as, the binominal formula.
      [1913 Webster]

   4. (Med.) A prescription or recipe for the preparation of a
      medicinal compound.
      [1913 Webster]

   5. (Chem.) A symbolic expression (by means of letters,
      figures, etc.) of the constituents or constitution of a
      compound.
      [1913 Webster]

   Note: Chemical formul[ae] consist of the abbreviations of the
         names of the elements, with a small figure at the lower
         right hand, to denote the number of atoms of each
         element contained.
         [1913 Webster]

   Empirical formula (Chem.), an expression which gives the
      simple proportion of the constituents; as, the empirical
      formula of acetic acid is C2H4O2.

   Graphic formula, Rational formula (Chem.), an expression
      of the constitution, and in a limited sense of the
      structure, of a compound, by the grouping of its atoms or
      radicals; as, a rational formula of acetic acid is
      CH3.(C:O).OH; -- called also structural formula,
      constitutional formula, etc. See also the formula of
      Benzene nucleus, under Benzene.

   Molecular formula (Chem.), a formula indicating the
      supposed molecular constitution of a compound.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Structural formula \Struc"tur*al form"u*la\ (Chem.)
   a symbolic representation of the structure of one molecule of
   a chemical compound, showing the attachments of the atoms to
   each other, and representing their arrangement in space; the
   spatial representation is symbolic and approximate, and it
   may or may not depict the stereochemical relations of the
   bonds. Distinguished from empirical formula. See also
   Rational formula, under Formula.
   [PJC]

WordNet (r) 3.0 (2006):

structural formula
    n 1: an expanded molecular formula showing the arrangement of
         atoms within the molecule