One of the most common definitions for efficiency in physics is a measurement of how much of the desired work or product is obtained from each unit of energy invested into that task or product. An additional definition of efficiency is based on the total amount of energy required to produce a specific amount of a product. Both definitions fall under the category of first law efficiency because they are derived from the first law of thermodynamics.
Some related terms and definitions regarding efficiency are electrical efficiency, mechanical efficiency and fuel efficiency. Although all of these involve a measure of usefulness, efficiency in physics is considered a technical or physical measurement. Those terms related to the accomplishment of goals or missions are usually defined by the terms efficacy and effectiveness.