An instrument that measures a surface's reflectance at many wavelengths, from which its colour under any light can be calculated.
It illuminates the sample with a known source in a defined geometry (commonly 45°/0° or diffuse/8°) and measures reflected light in narrow bands, typically every 10 nm across the visible range.
A colorimeter reports colour under its own fixed conditions; a spectrophotometer records the spectrum, so it can also predict metamerism.
Paint shops scan a sample with a spectrophotometer to formulate a match.
It is the reference instrument for colour matching and quality control in almost every industry that sells colour.