The Collaborative International Dictionary of English v.0.48:

Seminal \Sem"i*nal\, a. [L. seminalis, fr. semen, seminis, seed,
   akin to serere to sow: cf. F. seminal. See Sow to scatter
   seed.]
   1. Pertaining to, containing, or consisting of, seed or
      semen; as, the seminal fluid.
      [1913 Webster]

   2. Contained in seed.
      [1913 Webster]

   3. Hence: Holding the first place in a series of developed
      results or consequents; serving as a source, or first
      principle; giving rise to related ideas or results;
      germinal; radical; primary; original; as, seminal
      principles of generation; seminal virtue; a seminal
      discovery.
      [1913 Webster +PJC]

            The idea of God is, beyond all question or
            comparison, the one great seminal principle. --Hare.
      [1913 Webster]

   Seminal leaf (Bot.), a seed leaf, or cotyleden.

   Seminal receptacle. (Zool.) Same as Spermatheca.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Seminal \Sem"i*nal\, n.
   A seed. [Obs.] --Sir T. Browne.
   [1913 Webster]

WordNet (r) 3.0 (2006):

seminal
    adj 1: pertaining to or containing or consisting of semen;
           "seminal fluid"
    2: containing seeds of later development; "seminal ideas of one
       discipline can influence the growth of another" [syn:
       germinal, originative, seminal]

Moby Thesaurus II by Grady Ward, 1.0:

132 Moby Thesaurus words for "seminal":
   aboriginal, abundant, basal, basic, benchmark, blooming, bountiful,
   bursting, bursting out, central, cervical, clitoral, conceptive,
   conceptual, copious, creative, crucial, elemental, elementary,
   embryonic, esemplastic, excretory, exuberant, fattening, fecund,
   fecundative, fertile, fertilizing, flourishing, formative,
   fructiferous, fructificative, fruitful, fundamental, gametic,
   gamic, generative, generous, genetic, genital, germinal,
   ideational, ideative, imaginative, important, in embryo, in ovo,
   incipient, influential, ingenious, innovative, inspired, inventive,
   lachrymal, lachrymose, lacrimatory, lactational, lacteal, lacteous,
   landmark, life-giving, lush, luxuriant, multiparous, new, notional,
   original, originative, ovarian, penial, penile, phallic, plenteous,
   plentiful, potential, pregnant, primal, primary, prime, primeval,
   primitive, primordial, pristine, procreant, procreative,
   productive, proliferous, prolific, propagative, protogenic,
   radical, reproductive, rheumy, rich, richening, rudimentary,
   salivant, salivary, salivous, scrotal, secretional, secretive,
   secretory, serous, sexual, shaping, sialagogic, spermatic,
   spermatozoal, spermatozoan, spermatozoic, spermic, sporal,
   sporogenous, sporoid, sporous, superabundant, swarming, teeming,
   telling, testicular, thriving, uberous, undeveloped, unprecedented,
   uterine, vaginal, visioned, vulval, vulvar, watering, watery