Mucous is a describing word, and almost the only time you will meet it is in the phrase 'mucous membrane' — the soft, wet lining inside your nose, mouth, throat, eyelids and other openings of the body, which stays damp because it produces mucus. Mucus (spelled with -us) is the thick slippery liquid itself: the stuff that runs from your nose when you have a cold, and the stuff that quietly coats and protects your airways all the time. So mucus is the thing, mucous is the word you put in front of another noun. Both are ordinary medical vocabulary — a doctor or a biology textbook uses them without embarrassment — but in daily conversation English speakers avoid mucus and say 'snot' (childish and informal), 'a runny nose', or 'phlegm' for the thicker kind you cough up from your chest. Knowing the difference matters when you read a medicine box or a doctor's note.