Aristotelian definition , a standard approach, consists of placing a word in a category and then contrasting it with other members of the category. For example, a pencil is a type of writing instrument. It differs from a typewriter or word processor because it is held in the hand with no intervening machinery. It is different from a pen in that graphite rather than ink makes the mark. With concrete nouns, this type of definition is simple.
Operational definitions describe concepts which can be measured in some way.
For example, power can be defined operationally. A unit of horsepower is the amount of work necessary to do 33,000 foot-pounds of work in one minute of time. Concepts such as hot and cold can be defined in degrees Celsius or Fahrenheit. Contextual definition acknowledges the fact that words have different meanings depending on the situation or context in which they are used. The meaning of a word such as tart is not the same in a bakery as in reference to a human being, although historically the connection was obvious.