(n.) The C language interface to the X protocol.

This is the library of X-Windows functions distributed by MIT with the X Window system (hence, it's free -- just like X Windows). One can generate software that is compliant to the OpenLook or Motif look-and-feel (or any other, for that matter) using Xlib.

In Enhanced X-Windows, a C language subroutine library that client programs use to interface with the windowing system.

a collection of cryptographic services, such as user data encryption and decryption services