Thinking of what other people need before what you want, and acting on it. An unselfish person shares the last of something, gives up their seat, takes the worse shift so a colleague can go home. It is warm praise, but it is quieter praise than 'generous' or 'selfless': unselfish suggests a decent everyday habit of not putting yourself first, while selfless suggests forgetting yourself almost completely. Note the shape of the word — English built it by taking 'selfish' and cancelling it, so unselfish carries a faint sense of 'not doing the bad thing' rather than 'doing a shining thing'. Sport uses it constantly and with real force: an unselfish player passes to the teammate in the better position instead of shooting for personal glory, and coaches call that unselfish play. It is an ordinary, comfortable word — you can use it in speech and in writing without sounding stiff.