It indicates, in percentage, the ratio of incident light specularly reflected by the painted surface. It ranges between 0 (minimum gloss = no specular reflection of the incident light) and 100 (maximum gloss = specular reflection of the incident light); it should be noted that these values are purely indicative. Normally, the gloss degree of a coating is indicated with commercial terms, as follows:

Deep matt = 1 to 10 gloss degrees
Matt = 11 to 30 gloss degrees
Medium matt = 31 to 40 gloss degrees
Semi matt = 41 to 50 gloss degrees
Semi gloss = 51 to 80 gloss degrees
High gloss = from 81 gloss degrees on