登山って英語でなんて言うの？

"Mountaineering" is the name of the sporting activity where people climb mountains. A "mountaineer" is someone who climbs mountains. When you are walking up a mountain, this is called "trekking".

The act of going up a mountain is called mountain climbing or hiking.
Walking up a mountain is usually called hiking, but the act of climbing up the rocks of a mountain and being balanced with a rope is usually called mountain climbing.

-My friends and I usually hike on Saturday.
-I would like to go mountain climbing one day.

"Mountain climbing" is using your whole body to physically climb up the side of the mountain. "Trekking" on the other hand is hard/difficult hiking up and around the mountain, sometimes to the peak.

The act of climbing a mountain is mainly called 'mountaineering', but, it is also commonly called mountain climbing'. 'Mountain climbing' is often dangerous if the mountain has a lot of snow on it. Avalanches often occur and they can sweep and bury mountain climbers. Before climbing a mountain, it's important to find out if dangers such as this do exist on the mountain before starting on the journey so that a person can be better prepared.
So, you may say:
We will be going mountain climbing this summer.
or
I am now so used to mountaineering that it does not scare me anymore.

The two sentences you see above are great ways to express that you enjoy climbing mountains. The words hike and trek both mean to make a long, somewhat difficult journey on foot. These are words that would make great additions to your vocabulary, especially if you enjoy hiking or trekking a lot! I hope these two sentences and this explanation help you out.

Mountain climbing is the name given to the act of climbing mountains. When a person descends down a mountain and specifically has to make their way down a cliff side using ropes and pulleys, this is then called abseiling.