What is the X in X-mas?
mid-14c. (mid-13c. as a surname), agent noun from receive, or from Old French recevere (Modern French receveur), agent noun from recievere. As a telephone apparatus, from 1877; in reference to a radio unit, from 1891; in U.S. football sense, from 1897.
A device, as in a radio or telephone, that converts incoming radio, microwave, or electrical signals to a form, such as sound or light, that can be perceived by humans. Compare transmitter.