LF, in lieu of Cary's reply to your question, I believe "diffuser" is the final piece of the aeration system that divides up the main air flow into thousands of tiny bubbles. Many also call it the air stone.

Another analogy would be the shower head in your bathroom.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."