A theoretical definition defines a term in an academic discipline, functioning as a proposal to see a phenomenon in a certain way.
[1][2] Theoretical definitions contain built-in theories; they cannot be simply reduced to describing a set of observations.
[5] Social and health sciences interact with non-empirical fields and use both observation based and pre-existing concepts such as intelligence, race, and gender.
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".