This is the time in a man’s life, often after about fifty, when his body makes less of the hormone testosterone. Writers and doctors built the word as a male counterpart to menopause, the change women go through. Though actually it is not the same kind of clear stop: a man’s hormone levels usually fall slowly, not all at once, and not every man feels a big change. Some doctors find the word useful; others think it makes the change sound tidier than it is. You will meet it in health articles and clinics more than in ordinary chat. It is a fairly formal, medical-sounding noun.