热门 (rè mén) is a noun meaning 'a popular thing, hot topic, or something in vogue.' Unlike the adjectives 火 (huǒ) and 红 (hóng), which describe something as popular or in demand, 热门 refers to the popular item or topic itself (e.g., 热门话题 'hot topic'). It is similar to 热点 (rè diǎn) but broader: 热点 often implies a specific focus of attention (news, tourism), while 热门 can apply to any popular thing (majors, products).