Imprisoned is a recurring status effect in the Final Fantasy series.
Imprisoned is a status effect where a character is incapacitated by a special enemy attack. While imprisoned, they cannot take action until the enemy imprisoning them is defeated. Imprisoned characters are considered defeated. Therefore, if all three characters are Imprisoned, the game is over.
This status only occurs in three separate boss battles.
During the battle with Bottomswell in Junon, the Waterpolo attack will imprison a character. While imprisoned by the Waterpolo, their HP will trickle down as if they are being drowned. Damaging the Waterpolo with a magic attack will destroy it, freeing the character. It can also be countered by casting Regen on the imprisoned character, stopping the character's HP from dropping.
During the battle with Carry Armor, either Left Arm and Right Arm can imprison a character. When targeted, a character is picked up and held by one of the arms. During this state, damage inflicted on Carry Armor will reciprocate in damage inflicted on the imprisoned character(s). If an arm is destroyed, then the character that it is holding will be dropped, and a held character that dies will also be dropped.