The Collaborative International Dictionary of English v.0.48:

Sustenance \Sus"te*nance\, n. [OF. sustenance, sostenance,
   soustenance: cf. L. sustenentia endurance. See Sustain.]
   1. The act of sustaining; support; maintenance; subsistence;
      as, the sustenance of the body; the sustenance of life.
      [1913 Webster]

   2. That which supports life; food; victuals; provisions;
      means of living; as, the city has ample sustenance. "A man
      of little sustenance." --Chaucer.
      [1913 Webster]

            For lying is thy sustenance, thy food. --Milton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sustenance
    n 1: a source of materials to nourish the body [syn:
         nutriment, nourishment, nutrition, sustenance,
         aliment, alimentation, victuals]
    2: the financial means whereby one lives; "each child was
       expected to pay for their keep"; "he applied to the state for
       support"; "he could no longer earn his own livelihood" [syn:
       support, keep, livelihood, living, bread and
       butter, sustenance]
    3: the act of sustaining life by food or providing a means of
       subsistence; "they were in want of sustenance"; "fishing was
       their main sustainment" [syn: sustenance, sustentation,
       sustainment, maintenance, upkeep]