Lieutenant

Lieutenant is an ability word introduced in Commander 2014. It only appears on 2 horizontal cycles of 5 creatures each.

Description
Sometimes, highly competitive Commander players choose their commanders mostly for color identity, without the intent of building their decks around them. Lieutenant is meant as a reward for players who intend to cast their commanders and try to keep them on the battlefield.

A creature with lieutenant gets (or grants) a bonus as long as you control your commander. It allowed R&D to create creatures with combat stats that are more generous than they would normally allow for a typical one-on-one duel because their abilities will only be turned on in a context where players have a higher starting life totals. Moreover, unleashing your lieutenant's full power involves keeping your commander on the battlefield, which isn't always a given.

Rulings

 * Lieutenant abilities apply only if your commander is on the battlefield and under your control.
 * Lieutenant abilities refer only to whether you control your commander, not any other player's commander.
 * If you gain control of a creature with a lieutenant ability owned by another player, that ability will check to see if you control your commander and will apply if you do. It won't check whether its owner controls their commander.
 * If you lose control of your commander, lieutenant abilities of creatures you control will immediately stop applying. If this causes a creature's toughness to become less than or equal to the amount of damage marked on it, the creature will be destroyed.
 * If a triggered ability granted by a lieutenant ability triggers, and in response to that trigger you lose control of your commander (causing the lieutenant to lose that ability), that triggered ability will still resolve.

Other Use
Sometimes the protecting players in an Emperor game are also described as "lieutenants". However the proper descriptor would be "generals", which is ironically how many players call their commander.