No, "fissure" doesn't refer to a steep-sided mountain. It describes a narrow opening or crack in something, usually in the Earth's surface or in a rock formation.

Here's what a fissure is:

* Geology: A fissure is a crack or fracture in the Earth's crust, often caused by tectonic plate movement or volcanic activity.

* Other contexts: The word can also be used to describe a narrow opening in other materials, like a crack in a wall or a tear in fabric.

Steep-sided mountains are usually formed by different geological processes like uplift, erosion, or volcanic activity. They are called cliffs, peaks, or buttes depending on their size and shape.