A microconsole is a new low-cost, accessible and physically small game console. It's derived from the term "microcomputer". In case you're too young to remember them, microcomputers were cheap, easy-to-develop-for computers on which inexpensive games were sold. They had names like the BBC Micro, the Commodore 64, the Sinclair Spectrum and the Amiga (sort of) and were largely the reason why the "bedroom coder" came into being. They were also particularly popular in Britain, which is why so many old school legends of game development come from these tiny islands.
Microconsoles are very similar to microcomputers in many respects. For one thing, they are trying to make console gaming cheap and fun by being quite bare-bones in their offering. An Ouya is not really promising to be a high-powered media box, just a games machine selling games at app-prices that work on your TV. Microconsoles are also promising to be connected, and to be cheap and easy for developing games. While the main console providers pursue expensive and complicated (and largely failing) strategies to win the living room, microconsoles seem to want to get ahold of enthusiasts first and just give them what they want.