Undefended means without protection or guard. When a castle is undefended, no soldiers stand watch on its walls. When a goal in soccer is momentarily undefended, no goalkeeper or defenders block the net, giving attackers an easy chance to score.

The word appears often in military history. An undefended border between friendly countries has no troops or fortifications because the nations trust each other. An undefended city during wartime becomes vulnerable to attack because it lacks walls, weapons, or soldiers to protect it.

In sports, coaches look for undefended opportunities: the basketball player standing alone under the hoop, the hockey net left open when a goalie falls, the wide receiver running down the field with no one covering them. These undefended moments can change the outcome of a game.

The word can also describe ideas or positions. When someone makes a claim but offers no evidence or reasoning to back it up, their argument stands undefended. A good debater never leaves their position undefended because they know others will challenge weak points that lack support.