A topic is a subject. It's what you're discussing or what a newspaper article is about, the theme of a documentary, or the focus of your term paper.
Green energy, the Oscar nominees, what's on the lunch menu, boxers or briefs — these are all considered topics. If something is a "hot topic," then everybody is talking about it. If it's a "topic for discussion," then someone wants to have a serious conversation about it. If you're at a dinner party, you can talk about all kinds of topics with other guests. But to keep peace at the table, avoid bringing up controversial political or religious topics.
