1. something that transmits something (in all senses) 2. an electronic device that generates and amplifies a carrier wave, modulates it with a meaningful signal derived from speech, music, TV or other sources, and broadcasts the resulting signal from an antenna 
A transmitter (sometimes abbreviated XMTR) is an electronic device which with the aid of an antenna propagates an electromagnetic signal such as radio, television, or other telecommunications.
A transmitter usually has a power supply, an oscillator, a modulator, and amplifiers for audio frequency (AF) and radio frequency (RF). The modulator is the device which piggybacks (or modulates) the signal information onto the carrier frequency, which is then broadcast. Sometimes a device (for example, a cell phone) contains both a transmitter and a radio receiver, with the combined unit referred to as a transceiver - see also Radio transmitter design.
More generally and in communications and information processing, a "transmitter" is any object (source) which sends information to an observer (receiver). When used in this more general sense, vocal cords may also be considered an example of a "transmitter".
In industrial process control a "transmitter" is any device which converts measurements from a sensor into a signal to be received, usually sent via wires, by some display or control device located a distance away. Typically in process control applications the "transmitter" will output a 4-20 mA current loop or digital protocol to represent a measured variable within a range. For example, a pressure transmitter might use 4 ma as a representation for 50 psig of pressure and 20 ma as 1000 psig of pressure and any value in between proportionately ranged between 50 and 1000 psig. Older technology transmitters used pneumatic pressure typically ranged between 3 to 15 psig (20 to 100 kPa) to represent a process variable. 
In the context of The Teaching Mission, it is understood that transmitters (or TR's) are those human personalities offering the use of their mind and voice to give expression to celestial teachers who remain unseen. For a list of these TR's, follow this link.