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]

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "sustenance":
   TLC, aid, aliment, alimentation, alimony, backing, board, bread,
   bread and butter, care, carriage, carrying, cheer, chow,
   comestibles, continualness, continuance, continuation, continuity,
   creature comfort, cuisine, daily bread, eatables, eats,
   economic support, edibles, endowment, endurance, extension, fare,
   fast food, feast, food, food and drink, foodstuff, foodstuffs,
   groceries, grub, health food, ingesta, junk food, keep,
   kitchen stuff, lengthening, livelihood, living, maintenance, manna,
   meat, moral support, mothering, nosh, nourishment, nurture,
   nutriment, pabulum, pap, perpetuation, perseverance, persistence,
   price support, progress, progression, prolongation, protraction,
   provender, provision, provisions, psychological support, pursuance,
   rations, refection, refreshment, reliance, repetition, run, salt,
   scoff, security blanket, spread, staying power, straight course,
   subsidization, subsidy, subsistence, subvention, support,
   supportive relationship, supportive therapy, sustained action,
   sustaining, sustainment, sustentation, table, tender loving care,
   tucker, uninterrupted course, unremittingness, upholding, upkeep,
   viands, victuals, vittles, way