Hi SoHyun, When we walk up a mountain and come back down, we usually use the word “hike.” For example, “I’m going hiking tomorrow.” When we go on a very long walk, often for more than one day, and often up and down several hills or mountains, we use “trek.” For example, “Once I was trekking through the Andes...” When we need to use both our arms and our legs to get up a mountain, we use “climb.” For example, “Rock climbing is a new hobby of mine.” Then there are some other related words, such as “bouldering” (climbing on big rocks instead of mountains), “scrambling” (needing to use your arms and legs, but on flat ground, because the terrain is very rough) and “rambling” (another word for hiking, used more in British English). The section of a hike when you are going down the mountain is called the “descent.” You could use the verb “to descend” and it would be acceptable, but sounds a little too official for a hike with friends. Often we use the verb “to head up/down” meaning to move up or down. If you were at the top of the mountain and wanted to come down, you could say “shall we start heading down?” If you were leading a tour, you might instead say “shall we start the descent?” The degree that a hill or mountain goes up or down / \ could be called the “incline,” “slope” or “gradient.” If the incline is very extreme, we say that it is “steep.” For example, “That was a difficult hike, because the path was so steep.” If the incline is not extreme, and more flat, we say that it is “gentle” or “mild.” For example, “Don’t worry, it’s an easy hike. It’s gentle inclines all the way.” Hope this helps!