Specifically, the code defines a mapping between tri-nucleotide sequences called codons and amino acids; every triplet of nucleotides in a nucleic acid sequence specifies a single amino acid.
Because the vast majority of genes are encoded with exactly the same code, this particular code is often referred to as the canonical or standard genetic code, or simply the genetic code, though in fact there are many variant codes; thus, the canonical genetic code is not universal.
For example, in humans, protein synthesis in mitochondria relies on a genetic code that varies from the canonical code.