[mcpackieh]

FWIW, Galilean relativity aka Galilean invariance, the idea that Newton's laws are true in all inertial reference frames, predates that by a few centuries. But I'm not sure when they started using the word relativity for this.

[mdp2021]

The term 'relative' is Latin, and it is used for "reference" (of course: fero and latus are in the same verb).

Galieo uses the term 'relative', and to mean the judgement about magnitudes, but not for the invariance. The paragraph about the motion (the ships) does not contain it.

The term 'relativity' in English was coined by Coleridge; in Physics, adopted by Maxwell - see https://www.etymonline.com/word/relativity

In philosophy, as the parent mentioned, 'relativism' appears first in German in Wilhelm Traugott Krug (the successor of Kant), in 1838 - seemingly in a lexicographical work -, then in English in John Grote (1865) - see https://plato.stanford.edu/entries/relativism