"String" is a term used in computer science for a collection of symbols that have been selected from some underlying alphabet and ordered into a word. The length of a string is simply the number of symbols that it contains. For example, the length of the string "AAGATGCCGT" is 10.

In biology, strings are used to represent polymers, where each symbol represents a subunit of the polymer. Specifically, RNA, DNA, and proteins can all be represented by strings. RNA strings are formed from the alphabet {A, C, G, U}, DNA strings are formed from {A, C, G, T}, and protein strings are formed from a 20-symbol alphabet representing common amino acids; this alphabet excludes B, J, O, U, X, and Z.