Wiktionary
start codon
n. The codon at which Wikipedia:Translation (genetics) of mRNA into protein starts, encoding the first amino acid.
Wikipedia
Start codon
The start codon is the first codon of a messenger RNA (mRNA) transcript translated by a ribosome. The start codon always codes for methionine in eukaryotes and a modified Met ( fMet) in prokaryotes. The most common start codon is AUG.
The start codon is often preceded by a 5' untranslated region ( 5' UTR). In prokaryotes this includes the ribosome binding site.