Easing
Nothing in the world around us moves at a consistent speed. For example, when an apple falls from a tree, its speed increases with time because of how gravity works. To create such natural animations, designers use easing — an acceleration or deceleration. You can make an element move slowly at the start and then speed it up or vice versa. Easing makes moving objects look less artificial and robotic.
From Lesson
12 Principles of Motion Design