A theoretical definition defines a term in an academic discipline, functioning as a proposal to see a phenomenon in a certain way. A theoretical definition is a proposed way of thinking about potentially related events. Theoretical definitions contain built-in theories; they cannot be simply reduced to describing a set of observations. The definition may contain implicit inductions and deductive consequences that are part of the theory  A theoretical definition of a term can change, over time, based on the methods in the field that created it.

Without a falsifiable operational definition, conceptual definitions assume both knowledge and acceptance of the theories that it depends on.  A hypothetical construct may serve as a theoretical definition, as can a stipulative definition. The first postulate of special relativity theory that the speed of light in vacuum is the same to all inertial observers (i.e. it is a constant, and therefore a good measure of length). Of interest, this theoretical concept is the basis of an operational definition for the length of a metre is "the distance traveled by light in a vacuum during a time interval of 1/299,792,458 of a second". Thus we have defined 'metre' according to other ideas contained in modern scientific theory. 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[citation needed. Heat explains a collection of various laws of nature and that predict certain results. Differing theoretical definitions of "thinking" have caused conflict amongst artificial intelligence philosophers, illustrated for example by the different responses to the Chinese room experiment.[citation needed] Some philosophers might call "thought" merely "having the ability to convince another person that you can think".[citation needed] An operational definition corresponding to this theoretical definition could be a simple conversation test (e.g. Turing test). Others believe that better theoretical and operational definitions are required. The term scientific theory is reserved for concepts that are widely accepted. A scientific law often refers to regularities that can be expressed by a mathematical statement. However, there is no consensus about the distinction between these terms.[4] Every scientific concept must have an operational definition, however the operational definition can use both direct observations and latent variables.