There is no one fixed definition, except within specific games.

Also, many fighting games do not use that term at all.

This of course does not stop some people from using it like one ought to know WTF they mean.

It is generally short for “technical”, and often means a move that counters an enemy’s attack.

But it can mean other things in some games.

According to https://en.wiktionary.org/wiki/Appendix:Glossary_of_fighting_games :

“The term tech usually refers to “throw teching” (also commonly called Throw Escape or Throw Break) which is when a player counters and prevents the opponent’s throw attempt, usually by inputting their own throw. Although the end result of a throw tech can vary by game, both characters typicaly reel back a short distance from the broken throw attempt, and end up out of range of each other’s normal throws resetting the neutral.”

”[The term “tech” can also refer to quickly recovering from a knockdown attack.]”

“This term was first introduced following later versions of Street Fighter II, where it is also called “Softening” – this is slightly different from Throw Teching as it is conventionally known in more recent games. With Softening, the damage from being thrown is reduced instead of negated entirely, and while they still end up getting thrown after successfully Softening, the character receiving the throw lands on their feet instead of in a knockdown state.”

“Most games will only allow players to tech a normal throw, with the exception of the Tekken series which also allows players to tech command throws with the correct input(s).”

“Mortal Kombat 11 places importance on properly reading/teching forward & back throws via its new Krushing Blow mechanic; either one or both of certain characters’ throws will have a Krushing Blow that is triggered if the opponent incorrectly techs the previous throw.”