Stop Of Light
One doubling or one halving of the total light reaching the sensor, however that change happens — a wider aperture, a longer shutter speed, a higher ISO all move exposure by the same unit, which is exactly what lets them trade off against each other in equivalent combinations.
Aperture stops follow the less intuitive f/1.4, f/2, f/2.8 sequence rather than round numbers, since the f-number itself is a ratio and light gathered scales with the square of aperture diameter. See the stop of light answer page for the full aperture, shutter speed and ISO stop sequences laid out side by side.
Frequently Asked Questions
Why don't camera menus always move in full stops?
Most current cameras default to third-stop increments for finer exposure control, letting you nudge aperture, shutter speed or ISO in smaller steps than a full stop — the full-stop sequence is the reference scale, not the only available setting.
Is a stop the same thing as an EV (exposure value)?
They're numerically equivalent in practice — a one-stop exposure change and a one-EV change describe the same doubling or halving of total light, though EV specifically refers to a combined aperture-and-shutter-speed value calculated at a fixed reference ISO of 100.