I usually try to suppress my persnickety Mr. Language Person tendencies, but here I go….
“Defuse” and “diffuse” are different words. They’ve got a lot in common, because they each have precise physical/real-world meanings, though they’re both more often used metaphorically.
“Defuse” originally referred to removing the fuse from a bomb, rendering it harmless. Metaphorically, it refers to taking a (metaphorically) explosive situation and rendering it harmless. “Diffuse” originally referred to the way liquids or gases mix and spread out through each other, or the way substances can pass through a separating membrane. Metaphorically, it refers to taking something (possibly dangerously?) concentrated and spreading it out, making it less concentrated.
What’s the right thing to do when the room is full of concentrated and potentially explosive tension? Well, you could defuse the tension (so it won’t explode) or you could diffuse the tension (like a bad smell, as a way to “clear the air”). So yeah – the meanings overlap a bit. And since they do, the meanings of “defuse” and “diffuse” are slowly diffusing across the thin semantic membrane that separates them, and soon there won’t be any difference at all. That won’t stop me from griping though.
Rein/reign: the former has to do with horses, the latter has to do with kings. Watch Slate get it wrong in this sentence: “Astronomers reigned in Pluto because scientists were discovering other heavenly bodies that could qualify as planets, too …”. These astronomers are powerful over our notions of Pluto, but they’re not actually building castles there yet.
Now that I’ve opened myself up, bring on the spelling and grammar flames. 🙂