Nostalgia is a warm, slightly sad feeling you get when you think about a time that is gone and you liked it — an old school, a summer, the way your street used to feel. You are not only remembering; some part of you wishes that time had stayed, or that life still felt like that. People also use it for the act of sitting with those memories on purpose: old photos, old songs, the same stories told again. Though actually the word began as a doctor's name for homesickness. In 1688 Johannes Hofer built it in New Latin from Greek nostos, 'returning home', and algos, 'pain', to match German Heimweh, 'home-pain'. At first it was missing home so much it hurt. Later, helped along by French writing, it opened out to missing any past that feels sweeter than now. People stretch it a long way — last year, an old phone, even a 'look' they never lived through. It is not slang and not stiff; you hear it in ordinary talk, in ads that sell 'the old days', and in careful writing. It often paints the past kinder than it was, and that is part of how the feeling works.