The Collaborative International Dictionary of English v.0.48:

Inoculum \In*oc"u*lum\, n. (Microbiology)
   The preparation of microorganisms which is inoculated[5] into
   a growth medium; as, a small inoculum may not work well for
   the production phase of a fermentation.
   [PJC]

WordNet (r) 3.0 (2006):

inoculum
    n 1: a substance (a virus or toxin or immune serum) that is
         introduced into the body to produce or increase immunity to
         a particular disease [syn: inoculant, inoculum]