Amber codon refers to:

UAG the first codon to be deciphered was termed as amber codon. Mutants that are defective owing to the presence of an abnormal amber codon are called amber mutants. The other two stop codons are UGA and UAA. UGA is called the opal codon and UAA is called the ochre codon. Mutants that are defective because they contain abnormal opal or ochre codons are called opal and ochre mutants. Stop codons are called nonsense codons because they designate no amino acids. The initiating codon in mRNA is AUG and rarely GUG. Ref: Introduction to Genetic Analysis (Loose-Leaf) By Anthony J.F. Griffiths, page 330.