extender
/eɡstenˈdeɾ/ · B1 · verb
to extend, to expand
Latin ex- (out) + tendere (to stretch): stretch out, same root as extend
It's a cognate of English extend, plus the -er verb ending. It also means to spread: extender un mapa is to spread out a map, and extender la mantequilla is to spread butter. Extenderse sobre un tema means to expand on a topic.