To tick is to make a steady tapping sound. When something ticks — be it a clock, a watch, an oven timer, or a metronome — it’s usually a reminder that time is passing.
The verb tick originally meant "to touch or pat." The current meaning may be related to the sound of a hand patting or tapping a surface. Another kind of tick is a small blood-sucking parasite. This nastier tick has a different root, the Old English ticia.