means The principles of valid reasoning — the formal study of how conclusions correctly follow from premises, and by extension any sound, consistent line of thought.

from From Greek 'logikē,' short for 'logikē technē' — 'the art of reasoning' — built on 'logos,' that enormously load-bearing Greek word meaning 'word,' 'reason,' 'account,' or 'speech.' It traveled through Latin 'logica' and Old French before settling into English. The same 'logos' seeds a whole dynasty of '-logy' words, where it quietly shifted to mean 'the study of' — so biology, geology, and theology are all distant cousins of logic, each a discourse about something.