A man whose wife has died, and who has not married again. The word is only about death — a man whose marriage ended in divorce is not a widower, he is divorced. English is a bit unfair here: 'widow' (a woman whose husband has died) is the older, more common word, and 'widower' was built later by adding a piece to it, which is why the man's word is the longer one. You will meet it most often in serious or official writing — news reports, forms that ask about your marital status, obituaries, novels. In everyday talk people more often say the whole thing: 'his wife passed away a few years ago'. There is also a phrase 'golf widow' (or 'football widow'), a joke about a woman whose husband spends all his time on his hobby; the joke can be flipped to 'golf widower', but that is much rarer. Note too that a man stays a widower only until he remarries; after that he is simply a husband again, though people sometimes still say 'he was widowed young'.