Wintertime means the winter season, thought of as a stretch of time — in wintertime, during the wintertime. It says almost the same thing as winter, so the natural question is why anyone bothers with the longer word. The answer is feeling and rhythm. Wintertime sounds warmer and more old-fashioned than plain winter; it belongs to songs, stories, memories, and adverts ("a favourite wintertime drink", "back in the wintertime"). It also gently spreads the meaning out over the whole season rather than pointing at one cold period, so it fits sentences about what people usually do or how life usually is: In wintertime the lake freezes over. In plain, practical sentences, winter is the normal choice — you say last winter, not last wintertime, and This winter was cold, not This wintertime was cold. You will see it written as one word, and sometimes as two (winter time) or with a hyphen; one word is the safest. Watch out for one other use: in Europe, winter time can mean the clock setting used in the colder months, the opposite of summer time (daylight saving) — that meaning is usually written as two words.