What is the definition of response to stimuli?

It is something that causes a reaction in an organ or cell.

Generally, if we take the cellular response to stimuli, then it is defined as a change in state or activity of a cell in terms of movement, secretion, enzyme. In the nervous system, Stimulus-response coordination.

It could be smell, noise, or changes in blood sugar levels. 

For example, one can receive information from outside the body, as in touch receptors found in the skin or light receptors in the eye.