An effect type. A power that has the augmentable keyword has optional augmentations, which a character can use at the cost of power points. Only certain characters have power points, usually as a result of their class. Unless otherwise noted, using an augmentable power follows these rules.
Decide First: A creature must decide whether and how to augment an augmentable power when it chooses to use the power, before making any attack rolls or applying any of the power’s effects.
Power Point Cost: An augmentation specifies its cost in power points. For example, “Augment 1” means a creature must spend 1 power point to use an augmentation. The creature must spend the required power points when it decides to use the augmentation.
One at a Time: A creature can use only one augmentation on a power at a time, so it can’t, for example, spend 3 power points to use both a 1-point and a 2-point augmentation on a single power.
Replace Base Effects: When a power is augmented, changes to the power are noted in the description of the augmentation. If an augmentation includes a specific power entry, such as “Hit” or “Effect,” that entry replaces the entry in the base power that has the same name. An augmented version of a power is otherwise identical to the base power.
Unaugmented: When a creature uses an augmentable power without augmenting it, the power is referred to as unaugmented for that use (some effects apply only when a power is unaugmented). A power that doesn’t have the augmentable keyword is never considered unaugmented.
At-Will Attack Powers: When a power or some other effect lets a creature use an at-will attack power, the creature can choose to use one of its augmentable at-will attack powers, but must use it unaugmented. When a racial trait lets an adventurer choose an extra at-will attack power and the adventurer chooses an augmentable at-will attack power, the power loses both the augmentable keyword and its augmentations.
Published in Player's Handbook 3, page(s) 216, Rules Compendium, page(s) 115-116, 308.