The term responsive web design was first coined by Ethan Marcotte in his famous article Responsive Web Design, published in 2010 in A List Apart.
This Smashing Magazine article by Kayla Knight describes it as follows:
Responsive Web design is the approach that suggests that design and development should respond to the user’s behavior and environment based on screen size, platform and orientation. The practice consists of a mix of flexible grids and layouts, images and an intelligent use of CSS media queries. As the user switches from their laptop to iPad, the website should automatically switch to accommodate for resolution, image size and scripting abilities. In other words, the website should have the technology to automatically respond to the user’s preferences. This would eliminate the need for a different design and development phase for each new gadget on the market.