In computational complexity theory, a sparse language is a formal language (a set of strings) such that the complexity function, counting the number of strings of length n in the language, is bounded by a polynomial function of n. They are used primarily in the study of the relationship of the complexity class NP with other classes. The complexity class of all sparse languages is called SPARSE. Sparse languages are called sparse because there are a total of 2n strings of length n, and if a language only contains polynomially many of these, then the proportion of strings of length n that it contains rapidly goes to zero as n grows. All unary languages are sparse. An example of a nontrivial sparse language is the set of binary strings containing exactly k 1 bits for some fixed k; for each n, there are only Binomial coefficient strings in the language, which is bounded by nk.

Relationships to other complexity classes