The terms "terminal speed" and "terminal velocity" are often used interchangeably, but there's a subtle distinction between them. Here's the breakdown:

Terminal velocity refers to the vector quantity, meaning it has both magnitude (speed) and direction. So, terminal velocity is the constant speed an object reaches when falling through a fluid (like air) and the direction of that motion (usually downwards).

Terminal speed is the scalar quantity, referring only to the magnitude of the terminal velocity, meaning just the speed at which the object falls.

In essence:

* Terminal velocity is the full picture: speed and direction.

* Terminal speed is just the speed part.

However, in everyday conversation, people often use "terminal speed" and "terminal velocity" interchangeably. It's usually clear from the context what is meant.