Color Gamut
The range of colors a device or color space can reproduce. Wider gamut = more saturated colors possible.
Definition
A color gamut is the subset of the visible color spectrum that a display, printer, or color space can reproduce. sRGB covers about 35% of visible colors; DCI-P3 covers ~45%; Rec.2020 covers ~75%. Colors outside a gamut are 'clipped' to the nearest reproducible color, which is why a vivid red on your phone may look duller in a printed brochure.
Formula
Visualized as a triangle on the CIE 1931 xy chromaticity chart — the three corners are the device's R, G, B primary points. Larger triangle = wider gamut.
Example
An iPhone displays in DCI-P3, your office monitor likely in sRGB, professional printers in CMYK gamut (smaller than sRGB in the saturated cyan/blue range).