Defender is an example of a keyword that came about after the effect was used in Magic. Most commonly linked with the early 'Walls' of Magic it has seen more use in recent sets as a keyword that thematically means the creature is only good for getting in the way, not attacking.

702.3. Defender

• 702.3a. Defender is a static ability.
• 702.3b. A creature with defender can't attack.
• 702.3c. Multiple instances of defender on the same creature are redundant.

Rules Translation

• 702.3a. - Defender is always active
• 702.3b. - Defender just means that the creature can't attack.
• 702.3c. - Defender does not stack, it is all redundant