What is a receptor?

A receptor is an organ or cell which can respond to light, heat, or another external factor. They transmit an electrical impulse to the sensory nerve - this kick-starts the reflex arc.  There are many different kinds of receptors, all of which are specific to one kind of stimulus, they're all given names:

Mechanoreceptors - detect mechanical stimulus like pressure/vibrations

Thermoreceptors -  detect heat

Baroreceptors - detect high/low blood pressure in the heart

Chemoreceptors - detect chemical changes in the heart, so a change in oxygen, carbon dioxide or pH level

Fun Fact:

The retina of the octopus has up to 20 million light receptors :)