The energy or chemical that impinges upon and activates a sensory receptor is known as a "stimulus".
A major part of sensory receptors is to enable us to find out about nature around us, or about the condition of our inner condition or environment. Stimuli from fluctuating sources, and of various sorts, are gotten and changed into the electrochemical signs of the nervous system. This happens when a stimulus changes the cell film capability of a sensory neuron.