MIMO--Multiple Input Multiple Output
MIMO refers to the use of multiple antennas on both the transmitter or receiver to improve throughput and performance . It uses the phenomenon known as multipath (a RF signal bounces off walls and other structures and hits the antenna at different times) and organizes the mulitple occurences of the signal to achieve better performance.
In this MIMO primer the author describes a basic MIMO system which uses two, sometimes three antennas, to receive an RF signal. By measuring multiple signals over time, the signal processing in the system can reduce noise or interference from the multipath fading effect. It is used as part of the 802.11n specification to improve bandwidth.
Areas of future research into MIMO related to multi-user environments with multiple access schemes. Also, a great deal of research has yet to be implemented in hardware. For a further review, please check out this article.