Defensible means able to be defended or justified. A defensible position in a debate is one you can support with good reasons and evidence. If you claim that The Lion, the Witch and the Wardrobe is better than Charlotte's Web, that's a defensible opinion because you can point to specific reasons: the adventure, the magical world, the battles between good and evil. Someone else might disagree, but your position is defensible because you have real arguments backing it up.

In military contexts, a defensible location is one that can be protected against attack. A castle on a hilltop is more defensible than a camp in an open field because defenders can see enemies approaching, and the high ground gives them advantages.

The word also appears in law and ethics. A lawyer tries to show that her client's actions were defensible, meaning reasonable under the circumstances. When your parents ask why you stayed up past bedtime reading, “I only had three pages left in the chapter” is more defensible than “I forgot what time it was.”

The opposite is indefensible: impossible to justify or excuse. If you copied someone's homework word for word, that's indefensible. No good argument makes it okay.