Defuse or Diffuse?

Situation: Your two best friends have gotten into an argument over whose turn it is to buy beer (or Ensure, for those of you who are perhaps a little older and more health conscious). They go back and forth, becoming angrier with every breath. Eventually, when they are both too angry for words, the silent glaring begins. Tension builds. Will this be the end of a beautiful friendship? Of course not! Because you are there, ready to crack a joke and break the tension.

Now, when you crack that joke, are you defusing or diffusing the situation?

I am pretty sure I have never seen an author get this one right. I see it wrong in print all the time. I saw it wrong in print just this morning. I see it wrong so often, I have to look it up every time I come across it in my editing, just to be sure I’m not going crazy.

Defuse (the correct answer) is a verb meaning “1: to remove the fuse from (as a bomb) 2: to make less harmful, potent, or tense” (italics added).

Diffuse as a verb means “1: to pour out or spread widely 2: to undergo or cause to undergo diffusion 3: to break up light by diffusion.”

When you crack that joke, you are making the situation less harmful, potent, or tense; you are not pouring it out or spreading it widely or causing it to become diffuse. You are instead removing the fuse from the bomb that is about to go off in your buddies’ friendship. You are defusing the situation.

(Definitions from The Merriam-Webster Dictionary.)