Question

When is a body said to be at rest

Answer 1

rest is the state of a body which is stationary relative to a particular frame of reference or another object.  When the state of a body does not changes with time with respect to its surroundings in a particular frame of reference that time  it is said to be at rest.

in other words we can say that when a body has zero velocity and its position is not changing with time relative to its surrounding then it is said to be at rest.

For example a chair on which you are sitting is at rest or a table on which your desktop is kept is rest because its position is not changing.