For a physical line used to guide divers, see Guide line.
"Guideline" is the NATO reporting name for the Soviet SA-2 surface-to-air missile.

A guideline is a statement by which to determine a course of action. A guideline aims to streamline particular processes according to a set routine or sound practice. By definition, following a guideline is never mandatory. Guidelines are not binding and are not enforced. ( U.S. Dept. of Veterans Affairs, )

Guidelines may be issued by and used by any organization (governmental or private) to make the actions of its employees or divisions more predictable, and presumably of higher quality.

Examples of guidelines are: