Purulence is a noun. It names the state when a part of the body has pus in it, or is making pus — that thick yellow (sometimes green) liquid you see in a bad wound or a boil. Doctors write it in notes: they might say there is purulence, meaning the wound is wet with pus. Ordinary people almost never say this word. They say “pus,” or “it’s infected,” or “it’s oozing.” You will meet it in medical English, not in the street or in a café. Though actually, a few writers stretch it and talk about “moral purulence,” as if a person or a system were rotten inside. That use is rare and rather bookish. Learn the body sense first, and treat the other as a dark picture, not the main meaning.

化膿、有膿（醫學用語；日常多直接說「膿」）