Plenty of countries have legal definitions which are well enforced (1) and which often rely on internationally agreed upon standards (2).

Slippery slopes is usually a fallacious argument, decidedly so in this case where plenty of successful examples exists.

(1) https://en.wikipedia.org/wiki/R_v_Keegstra

(2) https://en.wikipedia.org/wiki/International_Covenant_on_Civi...