The Collaborative International Dictionary of English v.0.48:

Suet \Su"et\, n. [OE. suet, dim. fr. OF. seu, suif, F. suif, L.
   sebum. Cf. Soap, Sebaceous.]
   The fat and fatty tissues of an animal, especially the harder
   fat about the kidneys and loins in beef and mutton, which,
   when melted and freed from the membranes, forms tallow.
   [1913 Webster]

WordNet (r) 3.0 (2006):

suet
    n 1: hard fat around the kidneys and loins in beef and sheep