The Collaborative International Dictionary of English v.0.48:

Invention \In*ven"tion\, n. [L. inventio: cf. F. invention. See
   Invent.]
   [1913 Webster]
   1. The act of finding out or inventing; contrivance or
      construction of that which has not before existed; as, the
      invention of logarithms; the invention of the art of
      printing.
      [1913 Webster]

            As the search of it [truth] is the duty, so the
            invention will be the happiness of man. --Tatham.
      [1913 Webster]

   2. That which is invented; an original contrivance or
      construction; a device; as, this fable was the invention
      of Esop; that falsehood was her own invention; she
      patented five inventions.
      [1913 Webster +PJC]

            We entered by the drawbridge, which has an invention
            to let one fall if not premonished.   --Evelyn.
      [1913 Webster]

   3. Thought; idea. --Shak.
      [1913 Webster]

   4. A fabrication to deceive; a fiction; a forgery; a
      falsehood.
      [1913 Webster]

            Filling their hearers
            With strange invention.               --Shak.
      [1913 Webster]

   5. The faculty of inventing; imaginative faculty; skill or
      ingenuity in contriving anything new; as, a man of
      invention.
      [1913 Webster]

            They lay no less than a want of invention to his
            charge; a capital crime, . . . for a poet is a
            maker.                                --Dryden.
      [1913 Webster]

   6. (Fine Arts, Rhet., etc.) The exercise of the imagination
      in selecting and treating a theme, or more commonly in
      contriving the arrangement of a piece, or the method of
      presenting its parts.
      [1913 Webster]

   Invention of the cross (Eccl.), a festival celebrated May
      3d, in honor of the finding of our Savior's cross by St.
      Helena.
      [1913 Webster]

WordNet (r) 3.0 (2006):

invention
    n 1: the creation of something in the mind [syn: invention,
         innovation, excogitation, conception, design]
    2: a creation (a new device or process) resulting from study and
       experimentation [syn: invention, innovation]
    3: the act of inventing

Moby Thesaurus II by Grady Ward, 1.0:

178 Moby Thesaurus words for "invention":
   Nachtmusik, absolute music, adaptation, air varie, aleatory,
   aleatory music, apparition, arrangement, artifact, authorship,
   beginning, brainchild, breakthrough, bringing to light, bubble,
   canard, casual discovery, catching, chamber music,
   chamber orchestra, chance discovery, child, chimera, coinage,
   composition, conception, concoction, contraption, contrivance,
   contriving, creation, creative effort, creativeness, creativity,
   creature, crowning achievement, delirium, descant, detection,
   determination, determining, development, device, devising,
   disclosure, discovery, distillation, distinguishment, effect,
   eidolon, electronic music, end product, espial, essence, etude,
   excavation, exercise, exhumation, exposure, extract, extravaganza,
   fable, fabrication, fake, falsehood, falsification, fancy,
   fantasque, fantasy, fecundity, fertile mind, fertility, fib,
   fiction, figment, find, finding, finding out, forgery, fruit,
   gadget, generation, gizmo, hallucination, handiwork, harmonization,
   hatching, idle fancy, illusion, imagery, imagination, imagining,
   improvisation, incidental music, ingenuity, innovation,
   instrumental music, insubstantial image, introduction,
   inventiveness, issue, leap, lie, locating, location, lucky strike,
   maggot, make-believe, making do, manufacture, masterpiece,
   masterwork, mintage, myth, neologism, new mintage, new phase,
   nocturne, novelty, offspring, opera, opus, opuscule, orchestration,
   original, originality, origination, outcome, outgrowth, phantasm,
   phantom, piece, pregnant imagination, prevarication, product,
   production, productivity, program music, prolificacy, recognition,
   rediscovery, result, revelation, ricercar, romance, score,
   serendipity, sham, sick fancy, sonata, sonatina, spotting, story,
   strike, string orchestra, string quartet, study, tale, tall story,
   tall tale, teeming imagination, theme and variations,
   thick-coming fancies, treasure trove, trio, trip, trouvaille,
   trove, uncovering, unearthing, vapor, variation, vision, whim,
   whimsy, wildest dreams, work, yarn



Bouvier's Law Dictionary, Revised 6th Ed (1856):

INVENTION. A contrivance; a discovery. It is in this sense this word is used
in the patent laws of the United States. 17 Pet. 228; S. C. 1 How. U. S.
202. It signifies not something which has been found ready made, but
something which, in consequence of art or accident, has been formed; for the
invention must relate to some new or useful art, machine, manufacture, or
composition of matter, not before known or used by others. Act of July 4,
1836, 4 Sharsw. continuation of Story's L. U.S. 2506; 1 Mason, R. 302; 4
Wash. C. C. R. 9. Vide Patent. By invention, the civilians understand the
finding of some things which had not been lost; they must either have
abandoned, or they must have never belonged to any one, as a pearl found on
the sea shore. Lec. Elem Sec.  350.