The Collaborative International Dictionary of English v.0.48:

pesto \pes"to\ (p[e^]s"t[-o]), n. [It. pesto, pounded, from
   pestare to pound, crush, fr. LL. pistare, fr. pinsere. See
   also pestle.] (Cookery)
   A sauce used in Italian cuisine, typically made by blending
   olive oil, basil, garlic, pine nuts, and grated parmesan
   cheese; it is served hot or cold over pasta, meat, or fish.
   Where pine nuts are expensive, sunflower seeds are sometimes
   substituted.
   [PJC]

WordNet (r) 3.0 (2006):

pesto
    n 1: a sauce typically served with pasta; contains crushed basil
         leaves and garlic and pine nuts and Parmesan cheese in
         olive oil