Other forms: mountaineers; mountaineering
A mountaineer is a person who climbs or hikes mountains. If you live in Colorado and spend your weekends scaling peaks in the Rockies, you're a mountaineer.
The term mountaineer might seem to imply a serious climber, like Tensing Norgay and Sir Edmund Hillary, the mountaineers who were the first, in 1953, to climb to the summit of Mt. Everest. You can also use this word, however, for a more casual hiker who does a little rock climbing and mountain hiking. In many European countries, the word alpinist is more common.