A standard amino acid is an amino acid organisms use in the synthesis of peptides.  There are twenty standard amino acids, all of which are alpha amino acids.  Each standard amino acid has a three-letter symbol and a one-letter symbol.  Nineteen of the twenty standard amino acids are chiral, and only the L isomer (see D,L convention) occurs in eukaryotics.

eg:

see also nonstandard amino acid, side chain