What exactly is transmit power in wireless communication? My background is computer science. Recently, I had to come across some concepts in wireless communication/cellular mobile communication. Despite having gone through many materials, I am still having a hard time understanding some basic concepts, especially the concept of "transmit power". 

So, what exactly is transmit power? Is it:


the electrical power needed by a transmitter to transmit a signal to a receiver? 
or the amount of electrical power transmitted from a transmitter to a receiver via a channel?
or is it a different kind of power?
The transmit power for a single output transmitter typically refers to the output of the transmitter power amplifier that is connected to the transmit antenna, and that power is almost entirely radiated over the air. It is power in Watts.

The power needed by the transmitter includes this and everything else to run the transmitter. The power amplifier (PA) often dominates this power requirement which is why PA efficiency is such a big deal (For example, if a PA has a 50% efficiency and is transmitting 2W, it will require 4W of DC power to run). PA's are more efficient the closer we run them into saturation, hence constant envelope modulations such as FM (which can be saturated with no further distortion) are attractive when power efficiency is of prime importance.