What is an oscillation?

An oscillation is a term we could use to refer to a single cycle of a cyclic or repeating motion. Let's just take one example (so we don't run it into the ground) and see how it applies. And we'll use that familiar pendulum clock. The pendulum on a clock will be driven by a spring. We swing the pendulum up, release it, and gravity and the spring do the rest. The spring, as you have guessed, adds just enough energy to offset the friction loss in the system. When we lift the pendulum and release it, it is pulled down (accelerated) by gravity. It passes through a point where it is at the "bottom" of the swing (having achieved maximum velocity), and then heads up on the other side. It's decelerating here under the influence of gravity - that same gravity that accelerated it. I continues to slow, it stops, and then accelerates back down and across the "middle" or "bottom" again. Up it goes to the top of its arc on the other side, and it returns to its starting point. It has completed one cycle of its oscillation. It has performed one oscillation. Simple, and easy.