A theoretical definition gives the meaning of a word in terms of the theories of a specific discipline. This type of definition assumes knowledge and acceptance of the theories that it depends on.

For example, a theoretical definition of the length of a metre is "the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second". This is based on the first postulate of special relativity theory that the speed of light in vacuum is the same to all inertial observers, i.e. is a constant.

Theoretical definitions are common in scientific contexts, where theories tend to be more precisely defined, and results are more widely accepted as correct. The definitions of substances as various configurations of atoms are theoretical definitions, as are definitions of colors as specific wavelengths of reflected light. In such cases one definition of a term is unlikely to contradict another definition based on a different theory. However, in areas such as philosophy and the social sciences, theoretical definitions of the same term often contradict each other depending on whose theory is being used as the basis.

For example, in the noted Chinese room thought experiment, differing theoretical definitions of "thought" provide the main point of conflict. John Searle, author of the paper, challenges the definition of having "thought" as having the ability to convince another human (or, presumably, another being capable of thought) that it can think. Adherents to this idea believe passing the Turing test is an operational definition of thought. Searle rejects this theory, favored by some proponents of strong AI; in fact, his own theory maintains that it is impossible for a machine to be capable of thought by its very nature. Rejection of the theory underlying a theoretical definition leaves the definition invalid for use in argument with those who reject it -- neither side will advance its position by using terms the others do not accept.

A recent effect of differing theoretical definitions occurred when millions of Americans went from normal to overweight in a day's time, when the government standards changed to define overweight as a BMI over 25 which was based on new theories put forth by the National Institutes of Health rather than one of 27 for women and 28 for men, which reflected previous findings. Changing the definition of "overweight" put several million more in that category who hadn't gained a pound.