The simplest type of response is a direct one-to-one stimulus-response reaction. A change in the environment is the stimulus; the reaction of the organism to it is the response. In single-celled organisms, the response is the result of a property of the cell…
A stimulus is felt by an organism's senses which are sensitive parts of his physiology. ... When any part of an organism or of man's sensory organs is stimulated, a reaction or a response will result. A response is the behavior that is manifested by a living organism which is the result of an external or internal stimulus.