Domain

A protein domain is a sequence of amino acids that folds, 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, but can sometimes be shorter or longer.

Examples:

• 2hhd: Each of the 4 chains that form hemoglobin (a tetramer) folds into a single domain composed of alpha-helices. Each domain (chain) is 141-146 amino acids in length for human hemoglobin.
• 1igy: Immunoglobulin G (antibody) consists of 12 domains in 4 chains. These domains are composed of beta-sheets. Each of the two heavy chains has 4 domains, and each of the 2 light chains has 2 domains. These domains are about 110 amino acids in length. Each heavy chain has two pairs of domains connected by a flexible linker about 20 amino acids in length.

For more information see Protein Domain in Wikipedia.

See also and this summary of it.