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:
Average acceleration can be calculated as:
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).