Hash Function

A function that maps a bit string of arbitrary length to a fixed length bit string. Approved hash functions satisfy the following properties: 1) One-Way. It is computationally infeasible to find any input that maps to any prespecified output. 2) Collision Resistant. It is computationally infeasible to find any two distinct inputs that map to the same output.
SOURCE: SP 800-63; FIPS 201

A mathematical function that maps a string of arbitrary length (up to a predetermined maximum size) to a fixed length string.
SOURCE: FIPS 198

A function that maps a bit string of arbitrary length to a fixed length bit string. Approved hash functions are specified in FIPS 180 and are designed to satisfy the following properties: 1. (One-way) It is computationally infeasible to find any input that maps to any new prespecified output, and 2. (Collision resistant) It is computationally infeasible to find any two distinct inputs that map to the same output.
SOURCE: FIPS 186