A light defined by its spectrum on paper, so that colour calculations can say exactly which light they assume.
A table of relative spectral power by wavelength published by the CIE. A is incandescent tungsten, D65 and D50 represent daylight, and the F and LED series represent common lamp types. A real lamp only approximates one.
An illuminant is a specification; a light source is a physical lamp. 'D65 lighting' in a viewing booth is a source built to approximate the D65 illuminant.
The same CIELAB value is a different colour if it was computed under D50 instead of D65.
Every measured or calculated colour is only meaningful with its illuminant stated, which is why metamerism and print proofing depend on it.