Reactant

(n.) Substance that is consumed during a chemical transformation.

Receiver

(n.) An electrical component that transforms electrical energy. A lamp transforms electrical energy into light energy. A motor transforms electrical energy into mechanical energy.

Refrigerant

(n. and adj.) A substance that reduces temperature.

Renewable Energy Source

(n.) An energy source that renews quickly over the span of a human lifetime, such as water and wind.

Resistance

(n.) An electrical quantity denoted by R. Its unit is the ohm, symbolized by Ω (Greek letter omega). It is measured with an ohmmeter.

Resistor

(n.) An electrical component that allows more or less current to pass through.

Retina

(n.) A thin membrane at the back of the eye. It converts the light image into a nerve message transmitted to the brain.

Rod

(n.m.) Nerve cell located on the retina capable of detecting low-intensity light. These cells are also responsible for black-and-white vision. At night, the rods are active; colors are less distinguishable, but objects are seen in black and white, hence the expression "at night all cats are gray." The eye is a sufficiently refined instrument to detect a single photon.

Root Mean Square Voltage

(n.) In the case of a varying voltage, it is equal to the direct voltage producing the same average effects. It is measured with a voltmeter in alternating current mode.