The complete range of colours a particular device, ink set or colour space can produce or describe.
For an RGB space, the gamut is bounded by its three primaries: every colour it can encode is a mix of them. It is usually drawn as a triangle on a chromaticity diagram, though a true gamut is a three-dimensional solid that narrows towards black and white.
A bigger gamut is not automatically better. Content made for sRGB shown unconverted on a wide-gamut screen looks oversaturated, not more accurate.
A vivid screen green can sit inside Display P3 but outside sRGB, and outside the gamut of CMYK printing entirely.
Almost every 'it looked different on…' problem between screens, or between screen and print, is a colour meeting a smaller gamut and being moved to fit.