An alpha-amino acid has amino functionality on the carbon next to its carboxyl carbon. The skeleton in amino acid shows this adjacency explicitly. Numbering the carboxyl carbon as C1 makes the adjacent position C2, also called the alpha position. [ref]
Nomenclature and Symbolism for Amino Acids and Peptides (Recommendations 1983) by IUPAC-IUB Joint Commission on Biochemical Nomenclature. 3AA-1, Table 1 and its note on hypothetical uncharged formulas; 3AA-2.2.1, acyclic-amino-acid numbering
New to topics? Read the docs here!