Terminal velocity occurs during free fall when an object stops accelerating towards the ground, thus reaching a constant speed called terminal velocity. This is caused by air resistance. When the force due to air resistance is equal but opposite to the force by gravity, the object will reach terminal velocity.

You can see on my graph in Figure 3 green velocity line is curved, and it subtly gets less steep as time goes on. If this ball was able to fall infinitely below the level at which it was projected, the velocity curve would eventually flatten out so that its gradient is zero. This is the terminal velocity.