The UAG codon is a special kind of codon called a stop codon.
There are three types of stop codons: amber, ochre, and opal. UAG
is an mRNA codon that is specific for the amber stop codon. The
amber codon was named after Harris Bernstein, then a Caltech
graduate student, whose last name means "amber" in German. The
related link points to an article that discusses the history a bit
more.