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.