A body at rest may be defined as follows :
A body is said to be at rest, if it does not change its position with respect to a fixed point in its surroundings.
Or,
If the the position of a body remains unchanged with respect to any fixed point in its surroundings, then the body is said to be at rest.
For example -
A book laying on a table, if not disturbed by anybody, does not change its position with respect to the table, the walls of the room etc... So, a book laying on a table is at rest.
