The uterus is the organ inside a woman's body, and inside female mammals generally, where a baby grows from a fertilised egg until it is born. It sits low in the belly, is hollow and muscular, and it stretches enormously during pregnancy, then contracts strongly to push the baby out. It is a countable noun. The plural is 'uteruses' in ordinary use, though medical writing sometimes keeps the Latin 'uteri'. Say it 'YOO-ter-us', with the stress at the front. The adjective is 'uterine'.

English gives you two words for this one organ, and choosing between them is really the whole lesson. 'Uterus' is the clinical word — doctors, textbooks, scans, news reports about health. 'Womb' (say it 'woom'; the b is silent) is the older, warmer, everyday word, and the one you find in poetry, in the Bible, and in gentle conversation: 'a child in the womb'. Neither is rude; they simply carry different weather. A doctor explaining an operation says uterus. A grandmother does not.

Though actually, the split is not as neat as textbooks make it. English speakers have been getting more comfortable with the anatomical word — health campaigns, apps, and school lessons now use 'uterus' plainly where thirty years ago people reached for 'womb' or a vague phrase. So a learner will hear 'uterus' in far more everyday places than the formal label suggests. It also turns up outside medicine as a metaphor for a place where something is formed and protected before it emerges — but that use is literary, and 'womb' is the usual choice there.