<p>Two definitions of the velocity in translation mechanics or of the current in electromagnetism exist. A first one, often seen as a classical definition, states that these variables result from variations of distance or charge as a function of time. The second one, less classical, is based on the dependency of energy on momentum or induction flux. This paper discusses the properties that a system must have for using each definition. It is shown that the condition for involving time in a physical model is the existence of an energy conversion between two forms. The approach stems from the theory of Formal Graphs, in which the notion of elementary energy entity is introduced, with each entity containing a quantity of energy called energy-per-entity. The latter can be identical between several entities of the same nature, which then form a cohesive whole called a community of entities. There are as many such communities as there are different elementary forms of energy. These communities are the constituents of our macroscopic world, and are quantified by an extensive variable called the quantity of entities. The extensivity of such a state variable is the essential condition for it to fall within the scope of the first principle of thermodynamics, whose algebraic formulation enables all energies-per-entity to be expressed as partial derivatives of energy with respect to the corresponding quantity of entities. Moreover, these general definitions can be consistently extended to other state variables as this theory is transverse to all physics.</p>