Efficiency is the capability of acting or producing effectively with a minimum of waste, expense, or unnecessary effort. The term has widely variant meanings in different disciplines. See

• Algorithmic efficiency for its use in computer science.
  • efficiency is also a computer science term used when discussing parallel computing and speedup.
• Economic efficiency for its use in economics.
• Textual efficiency for its use in linguistics.
• Efficiency (statistics) for its use in statistics.
• Electrical efficiency for its use in electronics and electrical engineering.
• Fuel efficiency for its use in automobiles (also known as "gas milage").
• Mechanical efficiency for its use in engineering.
• In rocket propulsion, specific impulse is a measure for mass-efficiency, though a very high specific impulse is not energy-efficient
• Thermodynamic or "engine" efficiency for its use in Thermodynamics, Statistical Physics, related fields, and engines and motors.
• Volumetric efficiency for its use in engine design

In several of these cases efficiency can be expressed as a result as percentage of what ideally could be expected, hence with 100% as ideal case. This does not always apply, not even in all cases where efficiency can be assigned a numerical value, e.g. not for specific impulse.