The number of cycles a signal that occurs per second, measured in hertz (repetition rate). In electronics, almost invariably the number of times a signal changes from positive to negative (or vice versa) per second.

Only very simple signals (sine waves) have a single constant frequency; the concept of instantaneous frequency therefore applies to any transition, the frequency said to be the frequency that a sine wave making the same transition would have. Images have spatial frequencies, the number of transitions from dark to light (or vice versa) across an image, or per degree of visual field.