This is a term from Capcom vs SNK2, which is used when a player with only one character left alive manages to defeat all three opposing characters. This term seems to have been limited to this title, and has not been used much in other games of the same style such as Dragon Ball Fighter Z.
A 2-1 combo (short for two in one combo, also known as an interrupt combo or special cancel) is a combo that takes advantage of the fact that after some normal moves, the player can interrupt the animation to directly restart a move without waiting for the end of the animation, thus saving precious frames. However, this system is not present in all games.
As the name suggests, this is a full rotation of the stick, usually done in this direction: front-bottom-back-top. If it takes two turns, it is called a 720. Characters using these actions are usually choppers like Zangieff, and this manipulation gives access to a much more powerful chop than classic chops. Although it is considered a 360 move (the first in fighting game history), Zangieff's Spinning Piledriver can be performed with a 270° rotation instead of 360°. It can be done clockwise (front-down-back-up) or counter-clockwise (back-down-front-up). Many other games allow this flexibility for 360 attacks.
A kind of mix-up, where the defender has to choose between two defensive options that he cannot perform at the same time, leaving the attacker with a 50% chance of opening the guard. A well executed cross-up is a 50/50.
Holding the joystick or pad in one direction without changing it.