Moderately well delineated portion from which a whole is assembled.
For polymers, a subunit is the monomer. For polysaccharides this is a monosaccharide (or, more correctly, a monosaccharide moiety). For polypeptides the subunits are amino acids. For nucleic acids the subunit is a nucleotide.
The concept of subunits can apply, within biology, beyond the assembly of polymers. In particular, proteins can consist of more than one polypeptide unit, in which case the individual polypeptides are described as subunits. A multimeric protein thus consists of more than one subunit (and, indeed, can be described as possessing a quaternary structure which desribes inter-subunit interactions). As with polymers, the individual subunits making up multimeric proteins can either be identical or distinct or, indeed, some combination of the two.
Note that the word "subunit" can be viewed as translating directly into "sub" "unit" meaning that subunit is sub-portion of a larger unit, such as a polymer or proteins. The actually origin of the word subunit, however, is a bit more complicated.
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