means A well-tested explanation of how something works, built from evidence and reasoning rather than a single hunch — though in casual speech it can mean a mere guess.

from From Greek 'theōria,' meaning a looking at, viewing, or contemplation — itself from 'theōrein,' to look at or observe, related to 'theōros,' a spectator. The Greeks used it for the act of gazing intently at something to understand it, and a 'theōros' was originally an envoy sent to watch sacred games or consult an oracle. It passed through Late Latin 'theoria' into 16th-century English, where the sense narrowed from 'contemplative looking' to 'a reasoned scheme of how things work.'