[illumen]

Sometimes when people use terms they know the meaning behind them ;)

This is the original article which named the technique: http://alistapart.com/article/responsive-web-design/

Although the technique was being used earlier than May 25, 2010.

Adaptive Web Design (responsive but on server side, not client) is the other technique popular with websites aiming to support 96% of browsers well. Because it's impossible to do that nicely for mobile browsers.

Mobile first, offline first are the other two related terms you've likely heard. Meaning design for mobile first, or offline first.