An odour is a smell, and dictionaries are honest when they add “especially an unpleasant one.” That is how the word often leans in real life: the odour of smoke, the odour from the bins, body odour. Though actually, writers still use it for any smell they want to name with a slightly careful, slightly old voice — the odour of roses, the odour of rain on hot stone. It is the British spelling. American English writes odor and means the same thing. In speech, smell is the everyday word; odour sounds a step more formal, the kind of word you meet in notices, novels, and school English. Body odour is a set phrase (people shorten it to BO) for the smell a warm unwashed body gives off. If the smell is nice, scent and fragrance usually feel more natural. If it is clearly bad, stink and stench are stronger and ruder.