courgette (koor-ZHET) - n., the immature fruit of a vegetable marrow, called zucchini in North America.
However, I am North American, so I define it like this:
courgette (koor-ZHET) - n., (Brit.) zucchini, a green summer squash.
Zucchini being the name used in North America and Australia, courgette in the UK, New Zealand, and the rest of the Anglosphere. The US/UK distinction of squash/marrow is also of note. Borrowed around 1931 from French, of course, from the diminutive of courge, gourd, from Latin cucurbita, gourd/cup. Zucchini, meanwhile, was borrowed around 1929 from Italian, and is also a diminutive, in this case of zucca, gourd/squash, of uncertain origin but possibly from the same Latin root. From a Canadian news report:
A woman successfully fended off a bear attack in Montana with the only weapon she could find: a large zucchini also known as a courgette.