Domain
A protein domain is a sequence of amino acids that can fold, independently of the remainder of the full-length sequence, into a compact stable structure. Water-soluble domains typically have hydrophobic cores. Some small full-length proteins consist of a single domain, but most proteins have two or more domains. A domain is typically 100-250 amino acids in length[1], but can sometimes be shorter or longer.
For more information see Protein Domain in Wikipedia.
See also [1] and this summary of it.