THE purpose of the present note is to state the mathematical and physical definition of invariant in such a form as will make this notion generally applicable to observations ordered in time, and in particular applicable to economic variables. In mathematics we are taught to call an invariant a quantity which retains its numerical values in spite of transformations of coordinates. Take, for example, the length of a bar in a Euclidean space. If the coordinate system chosen is x, y, z, a Cartesian one, then the length of the bar in question will be