Nonconservative force
Also called Dissipative force, Non-conservative force
A force whose work depends on the path an object takes, so no potential energy function can be defined for it. Friction and air resistance are the examples the AP course names.
Work that depends on how you got there. The AP Physics 1 CED states that the work done by a nonconservative force is path dependent, and names friction and air resistance as its examples.
Path dependence rules out a potential energy. A potential energy has to be one value attached to one configuration, and it cannot be if two different routes to that same configuration cost different amounts. The CED says as much from the other direction: potential energies are associated only with conservative forces. There is no such quantity as friction potential energy.
The practical version turns up in the arithmetic. The energy dissipated by friction is typically equated to the force of friction times the length of the path over which the force is exerted, so you multiply by distance traveled rather than by displacement. Slide a block out across a rough floor and back to its starting point: gravity has done zero net work overall, while friction has taken a bite on both legs of the trip.
Mechanical energy is not conserved when one of these acts inside a system, but total energy still is. Whatever leaves the mechanical account arrives as thermal energy or sound.
One note on static friction. It has no potential energy either, but it dissipates nothing while the surfaces are not sliding, so it can appear in a problem without costing the system any mechanical energy.