It's called a cycle because it is one of each, not just a one direction thing. A cycle of a battery is defined as the full capacity of the battery drained and refilled again. That can be done many different ways, though. It's still just one cycle if you drain half and refill that half twice. Or a tenth ten times, etc. And not all cycles are created equal if you are doing this at various temperatures or doing this near the top or middle or bottom end of the battery, etc. There is a lot of research about it, but a cycle is a pretty simple definition.