The Collaborative International Dictionary of English v.0.48:

Alluvium \Al*lu"vi*um\, n.; pl. E. Alluviums, L. Alluvia.
   [L., neut. of alluvius. See Alluvious.] (Geol.)
   Deposits of earth, sand, gravel, and other transported
   matter, made by rivers, floods, or other causes, upon land
   not permanently submerged beneath the waters of lakes or
   seas. --Lyell.
   [1913 Webster]

WordNet (r) 3.0 (2006):

alluvium
    n 1: clay or silt or gravel carried by rushing streams and
         deposited where the stream slows down [syn: alluvial
         sediment, alluvial deposit, alluvium, alluvion]