Ah, this is a good question. Step into my time machine, and travel with me back to late eighties. The 3Com 3C501 PC Ethernet card was released at the bargain basement price of $999 USD and the mighty Interl 386-DX33 was released to great fanfare. Windows ran on top of DOS and all PC IP networking stacks were 3rd-party bolt-ons. When you communicated with a computer, whether DEC, IBM or whomever you did it with a DUMB TERMINAL. A terminal, for thos eof you who have never sern one, was either a keyboard and a printer combo (like a Decwriter 100) or a CRT text screen (Like a VT220, 80 columns by 24 lines) and a keyboard. What you typed went out the serial port to a computer, and the computer sent text and screen drawing back commands through that same serial port. That was the common human-computer interface of that era.
In the bad old days, the terminals were connected directly to the computers by serial interface cards and RS-232 cable. As computers got faster, more and more terminals could be supported. This started to get ridiculous as many serial lines emerged from these computers. The bigger computers looked like a mutant octopus.
Enter the TERMINAL SERVER, one of Cisco's early products. (The ASM)
The Cisco ASM terminal server took up to 96 terminals in an area and provided serial port connections for them. It then multiplexed those terminals over Ethernet in order to reach an Ethernet connected computer (host). The host computers were now freed from the sea of serial cables and could service dozens to hundreds of terminals thorough a single Ethernet port! As a result, dozens to hundreds of slow, lightly used 9600 bps serial cables were replaced by a single 10 Mbit/sec connection.
The Cisco ASM software provided a telnet client to each serial port so that each terminal could connect to any telnet server (host) on the any network. This allowed users to connect to multiple computers and relieved the load of computers providing telnet client services to directly attached terminals. In this model, connections were initiated from a terminal on the port of the Cisco ASM to some distant computer.
Over time, Cisco noticed that there were computers and other devices (like the IBM 7171) that could not support Ethernet conenctions, but had dozens upon dozens of physical serial ports. To make it so that network based telnet clients all over the Internet could access such an Ethernet-free computer, Cisco implemented a telnet SERVER function in the ASM. Connections from a distant telnet client could now conenct through the ASM telnet server and reach these serial ports which had previously been limited to local terminals or expensive modem connections. Since this activity was the exact reverse of the original use of the ASM, the function became known as "Reverse telnet."
In summary: When a serially attached device CONSUMES service FROM the network, that is TELNET. (The original design!)
When a serially attached device PROVIDES services TO the network, that is REVERSE TELNET. (The opposite of the original design. But useful!)
Now, I will wait for people who have been in the industry longer than I have to go on about SNA, 3270s, paper tape and punch cards.
