In computer science, a library is a collection of resources that is leveraged during software development to implement a computer program.

Historically, a library consisted of subroutines (generally called functions today). The concept now includes other forms of executable code including classes and non-executable data including images and text. It can also refer to a collection of source code.

For example, a program could use a library to indirectly make system calls instead of making those system calls directly in the program.