Exposure
The total amount of light that reaches the sensor or film, set by the combination of aperture, shutter speed and the scene's brightness.
Formula / rule
H = E × t
Measured in
lux-seconds (lx·s)
In depth
Exposure is a quantity, not an opinion: it is illuminance at the focal plane multiplied by the time the shutter stays open. ISO does not change how much light arrives — it changes how strongly that light is amplified — which is why photographers speak of 'exposure' and 'brightness' as two separate things. Every exposure decision trades one of these variables against another while keeping the product constant.
Examples in practice
A scene metered at f/8 for 1/125 s receives the same exposure at f/5.6 for 1/250 s: the aperture doubled the light per second, the shutter halved the seconds.