Definitions for "Start codon"
The codon on mRNA where polypeptide synthesis is initiated. The most common start codon is AUG but sometimes GUG or rarely UUG can be used as a start codon.
The mRNA triplet (AUG) that acts as signals for the beginning of translation at the ribosome. (Compare with stop codons. There are a few mnior exceptions to these codons.)
That codon at which translation of an mRNA molecule begins. This is always an AUG (encoding methionine) in eukaryotes, and nearly always in prokaryotes. In prokaryotes N-formyl-methionine is used to initiate polypeptide synthesis.