Multiple Input Multiple Output is a technique that utilizes multiple antennas at both the transmitter and the receiver, i.e. at both the base station and the terminal in a wireless network. Each antenna sends only a part of the total bit stream.
MIMO is especially useful in an environment with a lot of multipath fading i.e. in an environment where there are more than one transmission paths from the transmitter to the receiver due to reflections. This is especially the case in a mobile environment and in an in-house environment.
MIMO technology is used by both UMTS and Wi-Fi as a technology to increase the data capacity of a link.