If a body is uniformly accelerated, then it's velocity is changing by equal amounts in equal times. Suppose you drop a lead ball from a high tower. At the moment you drop it,
…its velocity is zero and it accelerates to 10 metres per second in the first second. In the fifth second for example it accelerates from 40 m/s to 50 m/s. The acceleation is ten more metres per second over each second. That is a constant acceleration. A steadily increasing velocity. In real life, the falling ball would not have constant acceleration because the faster it falls, the greater is the air resistance which tries to slow it down. If the ball falls from a high enough tower, its speed will become almost constant because the air resistance force upward will be close to the weight force downward and the acceleration will disappear. The ball reaches "terminal velocity". Linear motion is motion in a straight line, no sideways disturbances. (MORE) 
