Acceleration is the rate of change of velocity with respect to time. An acceleration in layman language can be defined as a phenomenon in which a person's speed changes constantly with time.

Acceleration of an object at an instant is given by:

$ a = \lim_{\Delta x\to\infty} \frac{\Delta v}{\Delta t} $

Average acceleration can be calculated as:

$ \Delta a = \frac{\Delta v}{\Delta t} $

If a person is under acceleration he/she can feel it in whatever direction they are accelerating. The person's body will lean against the direction of acceleration. For example, a person driving a car starts to accelerate it at 2 m/s2. The driver's body will feel the force of acceleration and lean backwards. His body leans backwards i.e. against the direction of force because it has a tendency to remain at rest (Inertia of rest).