My writing teacher has been using the word cogency all the time, and I never bothered to look it up. I’ve been so busy with school, I haven’t had time for anything else!
But anyway, all complaining aside, the word cogency means “the quality or state of being convincing or persuasive”. The adjective cogent is more common than the noun, and it means “convincing or believable by virtue of forcible, clear, or incisive presentation”. So far, I don’t think my college essays have been particularly cogent. If only I looked up the word sooner…
The Latin root of cogent is “cogere ‘to curdle, to compel, to collect,’ from com- ‘together’ + agere ‘to drive’”, according to the Online Etymology Dictionary. That Latin word also led to the English words coagulate, cache and, strangely enough, squat.