The Collaborative International Dictionary of English v.0.48:

Formation \For*ma"tion\ (f[o^]r*m[=a]"sh[u^]n), n. [L. formatio:
   cf. F. formation.]
   1. The act of giving form or shape to anything; a forming; a
      shaping. --Beattie.
      [1913 Webster]

   2. The manner in which a thing is formed; structure;
      construction; conformation; form; as, the peculiar
      formation of the heart.
      [1913 Webster]

   3. A substance formed or deposited.
      [1913 Webster]

   4. (Geol.)
      (a) Mineral deposits and rock masses designated with
          reference to their origin; as, the siliceous formation
          about geysers; alluvial formations; marine formations.
      (b) A group of beds of the same age or period; as, the
          Eocene formation.
          [1913 Webster]

   5. (Mil.) The arrangement of a body of troops, as in a
      square, column, etc. --Farrow.
      [1913 Webster]

WordNet (r) 3.0 (2006):

formation
    n 1: an arrangement of people or things acting as a unit; "a
         defensive formation"; "a formation of planes"
    2: the act of fabricating something in a particular shape [syn:
       formation, shaping]
    3: the act of forming or establishing something; "the
       constitution of a PTA group last year"; "it was the
       establishment of his reputation"; "he still remembers the
       organization of the club" [syn: constitution,
       establishment, formation, organization, organisation]
    4: (geology) the geological features of the earth [syn:
       geological formation, formation]
    5: a particular spatial arrangement
    6: natural process that causes something to form; "the formation
       of gas in the intestine"; "the formation of crystals"; "the
       formation of pseudopods"
    7: creation by mental activity; "the formation of sentences";
       "the formation of memories"

Moby Thesaurus II by Grady Ward, 1.0:

186 Moby Thesaurus words for "formation":
   Platonic form, Platonic idea, accumulation, acronym, acrostic,
   aesthetic form, allocation, allotment, anatomy, appearance,
   apportionment, archetype, architectonics, architecture,
   arrangement, array, arraying, art form, assemblage, assembly,
   avyayibhava, back formation, build, building, buildup, cast,
   casting, clipped word, close formation, collation, collocation,
   combination, composition, compound, concord, configuration,
   conformation, conjugate, constitution, construction, conversion,
   crafting, craftsmanship, creation, crystallization, cultivation,
   cut, deployment, design, development, devising, display, disposal,
   disposition, distribution, dvandva, dvigu, echelon, effectuation,
   efformation, elaboration, embodiment, endocentric compound,
   erection, establishment, exocentric compound, extraction, fabric,
   fabrication, fashion, fashioning, figuration, figure, forging,
   form, format, formation flying, formature, forming, formulation,
   foundation, founding, frame, framing, generation, genesis, genre,
   getup, grouping, growing, handicraft, handiwork, harmony,
   harvesting, impression, inauguration, inception, incorporation,
   inner form, installation, institution, junction, layout, lineup,
   machining, make, makeup, making, manufacture, manufacturing,
   marshaling, materialization, matrix, milling, mining, mixture,
   modality, mode, model, modeling, mold, molding, morphogenesis,
   morphogeny, order, ordering, organic structure, organism,
   organization, organizing, paronym, pattern, patterning, peace,
   physique, piecing together, placement, plan, prefabrication,
   preparation, processing, producing, production, proportion,
   prototype, putting together, quiet, quietude, raising, realization,
   refining, regimentation, regularity, routine, sculpture, set,
   set-up, setting-up, setup, shape, shaping, significant form,
   smelting, spoonerism, stamp, structure, structuring, style,
   symmetry, syneresis, syntax, synthesis, system, tatpurusha,
   tectonics, texture, tissue, tranquillity, turn, type, uniformity,
   warp and woof, weave, web, word form, workmanship