What does the term terminal velocity mean?

Terminal velocity is the constant speed (in a particular direction) a body reaches when the forces are balanced, i.e. acceleration = 0 m s^-2.

An example of this is a ball bearing dropped in oil.

⬆ Resistance (R)

O

⬇ Weight (W=mg)

Explanation: At first the forces are unbalanced and W > R so the ball accelerates downwards. The faster the ball moves the greater R is, so R is increasing. A point is reached where R = W. There is now no net force acting on the ball so it moves at a constant velocity (Newton's first law). This is the terminal velocity.