I've got the job to hire three terminators for my team.
The only problem is that I've no clue about what a terminator is.

Hope someone can help me.

Terminator is a level of the 'Attitude' Mental Characteristic. It's what would be the +1 setting. It means that the player will have less fitness deduction in game.
(read the manual for further information)

Where do I find these players then??

Well, it shows in the player's profile. You can't search for it, you just have to check the profiles of the players you are interested in.