A thrill-seeker is a person who goes looking for excitement, especially the frightening kind — jumping out of planes, riding the biggest roller coaster, driving fast, climbing without ropes. The point is the feeling: that rush of fear-mixed-with-joy in the chest and stomach. A thrill-seeker chases that feeling on purpose, and the danger is not an accident, it is part of what they came for. The tone sits somewhere between admiring and worried, and it moves with who is speaking. A travel advert will say 'perfect for thrill-seekers' as pure praise. A parent or a news report will use the same word with a shake of the head, meaning someone who takes stupid risks. It is most often used of people doing sports and rides, but it stretches: you can call someone a thrill-seeker for chasing risky investments or dramatic relationships. You will see it written with a hyphen, as one word ('thrillseeker'), or as two — all three are around, and the hyphen is safest. The behaviour itself is 'thrill-seeking', which works both as a noun and as an adjective: 'thrill-seeking behaviour'.