Simple Definition:

Terminal velocity is the maximum speed that an object can reach when falling through a fluid [a substance, like air or water, that can flow]. It occurs when the force of gravity pulling the object downwards is balanced by the drag force [the force that opposes the motion of an object through a fluid], causing the object to fall at a constant speed.

For example, when a skydiver jumps out of an airplane, they initially accelerate [increase their speed] due to gravity. However, as they fall faster, the drag force from the air also increases. Eventually, the drag force becomes equal to the force of gravity, and the skydiver reaches terminal velocity, falling at a steady speed without further acceleration.

Very Simple Definition:

Terminal velocity is the fastest speed something can go when it falls through the air. It’s like when you throw a paper plane and it stops speeding up and just floats down.