What is the term for the force that opposes motion through a fluid?

The term that describes the force opposing motion through a fluid is drag. In fluid dynamics, drag is a resisting force that occurs when an object moves through a liquid or gas. This force arises due to the interaction between the object and the fluid, which can be influenced by factors such as the object's shape, surface texture, and the fluid's speed and density.

For example, when an airplane moves through the air, drag acts against the thrust generated by the engines, requiring the engines to work harder to maintain speed. Similarly, a boat moving through water experiences drag, which affects its speed and fuel efficiency. Understanding drag is crucial in engineering, especially in fields like aerodynamics and hydrodynamics, as it directly impacts the design and performance of vehicles and structures.