a fighter who holds out against attack
"The loyal defender held out against the overwhelming attack until reinforcements arrived."
someone who defends people or property
"The knight acted as a defender, standing between the villagers and the invading army to protect their homes."
In plain English: A defender is someone who protects others from harm or attacks them on their behalf.
"The goalkeeper was the main defender protecting his team's net from scoring goals."
The word "defender" entered English in the Middle Ages through a mix of borrowings from Anglo-Norman and Old French as well as native formation by adding "-er" to the verb "defend." It originally referred to one who protects or guards, which remains its core meaning today.