CIS is short for "Character Induced Stupidity", it refers to mental limits of a character that cause them to perform at below the true potential of their powers. The term is somewhat of a misnomer, since it refers to more than just stupidity - it could also refer to a character's unawareness, morals, or any other mental characteristic that limits the use of their powers.
Examples of CIS:
- Martian Manhunter not mind haxing instantly due to his relationship with the justice league
- Vegeta not killing Cell then deciding to let him to absorb Android 18 and turn into his Perfect form since he wanted a stronger opponent.