In its most basic form, the Defend command is a way of reducing incoming damage. A defending Adept cannot take any action that turn, however, all incoming offensive damage is reduced by 50%. The effects of Defend last the entire turn, and and thus ignores the user's (or enemy's) agility rating. Defending does not reduce your chances of being inflicted with a status affecting attack, as those attacks are affected solely by Luck. However, it does also cut in half any damage done to your PP meter. Several enemies also have the ability to defend. For more information, see Enemy Abilities.
The Defend command's mechanics vary by game. In Golden Sun, and Golden Sun: The Lost Age, the Defend command can be chosen, but does not have any unique animations. However, when an enemy is downed prior to other Adepts taking their turn, any Adepts ordered to target the fallen enemy are instead automatically ordered to Defend. In the Battle Mode's Link Battles, any Adept that is not given a command by the end of a specified time limit is automatically placed into Defend. These mechanics were actually the cause of a lot of negative remarks in the reviewer community, and were described as a lack of "smart combat," as said by an IGN reviewer.
In Golden Sun: Dark Dawn, the Defend command is described in-game as "Keep[ing] your guard up." Changes to the battling system now mean that an Adept targeting an invalid target (for instance, attacking an enemy that was felled earlier in the turn, or healing an ally that has downed) will no longer be set to defend, rather, they will swap to a new, valid, target. Adepts ordered to Defend assume a defensive stance, usually with a weapon held in front of them. A special case emerges when Sveta enters the Wild Animal class series. Instead of the standard Defend command, Sveta will have access to the similar Pack Defense command. Pack Defense grants all party members the effects of Defending, while still being able to perform their normal attacks. In this way, Pack Defense can be considered an equivalent to Shielding Djinn like Granite.
As a command, Defend is often an undesirable option to use in battle, as a defending Adept cannot perform any other action and thus will not be able to aid the party on that turn. While the effect (reducing the rate of HP loss) is useful, there are usually alternatives, as increasing the rate of healing is almost always a much more effective tactic as healing items and Psynergy are plentiful and, with a few exceptions, will heal much more than 50% of an enemy's attack.
However, when facing bosses – or even strong enemies – and there are no other useful actions that the party member can perform (for example, if all defensive Djinn are on standby) then Defend is a viable option. In Dark Dawn, the similar Pack Defense may see more use, as the command functions as a shielding Djinn equivalent, without the hassles of spending turns Setting and Unleashing.
If, however, the objective is to not win the fight as efficiently as possible, but rather to win it in a specific way – for example, via a method that utilizes the Random Number Generators of Golden Sun and The Lost Age to ensure a 100% drop rate on rare items – then defend is a very handy option, as an inactive Adept does not add to the "Random Numbers", allowing a slower Adept to use an appropriate Djinni to fell the enemy and complete the sure-drop method.