A rule is:

• Rewrite rule, in generative grammar and computer science
• Standardization, a formal and widely-accepted statement, fact, definition, or qualification
• Operation, a determinate rule (method) for performing a mathematical operation and obtaining a certain result (Mathematics, Logic)
  • Unary operation
  • Binary operation
• Rule of inference, a function from sets of formulae to formulae (Mathematics, Logic)
• Rule of thumb, principle with broad application that is not intended to be strictly accurate or reliable for every situation. Also often simply referred to as a rule
• Moral, an atomic element of a moral code for guiding choices in human behavior
• Heuristic, a quantized "rule" which shows a tendency or probability for successful function
• A regulation, as in sports
• Procedural law, a ruleset governing the application of laws to cases
  • A law, which may informally be called a "rule"
  • A court ruling, a decision by a court
• In the U.S. Government, a regulation mandated by Congress, but written or expanded upon by the Executive Branch.
• Norm (sociology), an informal but widely accepted rule, concept, truth, definition, or qualification (social norms, legal norms, coding norms)
• Norm (philosophy), a kind of sentence or a reason to act, feel or believe
• "Rulership" is the concept of governance by a government:
  • Military rule, governance by a military body
  • Monastic rule, a collection of precepts that guides the life of monks or nuns in a religious order where the superior holds the place of Christ

Other uses

• Rule (song), a song by rapper Nas
• Ruler, an instrument for measuring lengths
• Rule, a component of an astrolabe, circumferator or similar instrument
• Rule of thumb, Acquisition, a set of guidelines intended to ensure the profitability of businesses owned by the ultra-capitalist Ferengi in the fictional Star Trek universe
• The Rules, a bestselling self-help book
• RULE Project (Run Up-to-date Linux Everywhere) is a project that aims to use up-to-date Linux software on old PCs
• Rule engine, a software system that helps managing business rules
• Ja Rule, a hip hop artist
  • R.U.L.E., a 2005 greatest hits album by rapper Ja Rule