A nucleotide is an organic molecule consisting of a nitrogenous heterocyclic base (a purine or a pyrimidine), a pentose sugar (deoxyribose in DNA or ribose in RNA), and a phosphate or polyphosphate group.

A nucleoside is similar, except that it contains only the sugar and base, without a phosphate.

Nucleotides are the monomers of nucleic acids and also play important roles in cellularular energy transport and transformations (notably ATP and NAD+/NADH) and in enzyme regulation (see for example, protein kinase).