'Summit' is Latin for 'the highest point' — from 'summus' (highest). The diplomatic sense came later.
The highest point of a hill or mountain; the highest attainable level of achievement; a meeting of heads of state.
From Old French sommet ("summit, top"), a diminutive of som ("top, peak"), from Latin summum ("the highest point"), neuter of summus ("highest"), a superlative form from PIE *upo- or *sup- ("up, over, above"), with the superlative suffix *-mo-. The PIE root *upo- is the ancestor of an enormous family: Latin super ("above"), sub ("under" — originally "up close to"), Greek ὑπέρ (hypér, "over") and ὑπό (hypó, "under"), Sanskrit úpa ("near, toward"), and English up, over, and upon. The Latin superlative summus (from earlier *sup-mos, "uppermost") followed