What is an abstract noun for fins?

There isn't a single perfect abstract noun for "fins." However, depending on the context, you could use:

* Locomotion: This refers to the act of moving, and fins are used for swimming, a form of locomotion.

* Propelling: This emphasizes the force fins generate to move through water.

* Hydrodynamics: This refers to the study of how fluids move and interact, and fins are essential for efficient movement in water.

* Streamlining: This describes the smooth, sleek shape of fins that minimizes resistance in water.

The best abstract noun to use will depend on the specific meaning you want to convey.