A triplet of nucleotides that codes for one amino acid is called a codon.

Here's a breakdown:

* Nucleotides: The building blocks of DNA and RNA. There are four main types: adenine (A), guanine (G), cytosine (C), and thymine (T) in DNA, or uracil (U) in RNA.

* Codon: A sequence of three nucleotides that specifies a particular amino acid.

* Amino Acid: The building blocks of proteins. There are 20 different amino acids commonly found in proteins.

For example, the codon AUG codes for the amino acid methionine.

There are 64 possible codons (4 nucleotides x 4 nucleotides x 4 nucleotides = 64), but only 20 amino acids. This means that some amino acids are coded for by more than one codon, which is called degeneracy or redundancy.