means A medium-grade metamorphic rock that splits easily into thin layers, its surface glittering with aligned mica and other platy minerals.

from From French 'schiste,' which came from Latin 'schistos' and ultimately Greek 'skhistos,' meaning 'split' or 'divided' — from the verb 'skhizein,' 'to split' (the same root that gives us 'schism' and 'schizo-'). The Greeks named it for exactly what it does: cleaves cleanly along its layers.