An everyday word for how light-emitting or luminous a colour looks — often used loosely for both value and lightness.
Not a single precise colour-science term on its own; usually refers to HSV value, though it is sometimes used for perceptual luminance (a physically measured quantity related to, but distinct from, either HSL lightness or HSV value).
'Brighter' in Adjust My Colour specifically means an HSV-value increase, not a saturation increase — 'brighter' and 'more vivid' are not the same adjustment.
Adjust My Colour's 'brighter' option raises HSV value; it will not necessarily make a colour look more saturated.
Knowing which technical meaning is in play avoids confusion when a tool's 'brighter' result looks different from what was expected.