It's where you rely on specialists for your science. This normally involves having lots of food sources and farms in your cities to support said specialists, and Representation from Pyramids/Consitution for extra beakers per specialist. A pure specialist economy isn't very efficient IMO, because you will need much bigger cities to support them and so increased maintenance, you need Caste System early if you want loads of scientists/merchants early (which means forgoing the awesome Slavery), and the fact that you need lots of excess food and fresh water for farms, making such an economy highly dependent on your surrouning land.
That being said, a hybrid economy is best; allowing you to run specialists in high-food areas and cottages everywhere else. Also helpful for late-game conquests post-Biology, which allows you to run a 6-beaker scientist per (farmed) grassland, which is much more efficient than waiting for cottages to mature that late in the game.
