A manifestly covariant definition of the linear potential is applied to a general class of relativistic theories. It is shown that in momentum space this potential exists in a distributional sense, leading to well-defined equations. Therefore no extra parameters are needed to avoid singularities. Retardation effects, whose precise form depends on the details of the theory under consideration, can easily be included. To investigate the importance of these retardations for charmonium, this linear potential is used in combination with a Coulomb potential for a system of two spinless particles. The calculations are performed within a framework introduced by Ruijgrok and de Groot [for a review see, e.g., Phys. Rev. A 46, 3657 (1992)]. This theory was proven to give the proper lowest-order relativistic corrections. The retardation effects are equally important as the other relativistic corrections.