Defensive abilities can be mastered to 3 different tiers; Better, Greater and Superior.
This is the third of the three tiers.

When payment is made to improve a defensive ability, the ability will increase only by a percentage, anywhere from 15% - 20%.  Once 100% mastery has been achieved, the ability will begin the next tier.  The purchase price, upkeep cost and improvement cost all increase with each tier.  See the Ability Mastery Chart for breakdown off all ability improvement stats, or the links on the right for each individual category.

When abilities are viewed through the Stats page, the type and number of abilities used when "attacking" are highlighted.  These highlighted abilities are not necessarily the same abilities you are using when you defend your clan from "being attacked".

When abilities are viewed through the Bazaar page, the attack and defense strength are shown as well as the number of each particular ability you own.

Each ability has it's own attack and defense strength.  The highest attack strength will be used first and then down the line when attacking.  The highest defense strength will be used first and then down the line when defending from attack.