A single number for how different two colours are.
A distance calculated in CIELAB. Several formulas exist — CIE76, CIE94, CMC and CIEDE2000 — and they give different values for the same pair, so a ΔE figure means little unless the formula is named.
There is no universal 'ΔE 2 is acceptable'. Tolerances depend on the formula, the material, the viewing conditions and the industry.
The same two blues can measure ΔE*ab 4.0 and ΔE00 2.0.
It is how colour matching is specified and checked in paint, print, plastics and textiles.