What is Diversity in term of MIMO

take a look at this book: Fundamentals of Wireless Communication by david tse. It is freely available on the internet:

http://www.eecs.berkeley.edu/~dtse/book.html

diversity means that your antenna can operate at several frequency.
we have some kind of diversity.
SPACE diversity.
FREQUENCY diversity

diversity are used in wierless communication.

diversity: If you have more than one antenna at the receiver, there is a very good possibility that when one antenna gets almost no signal (deep fade), one of the other antenna gets better signal. Therefore, you receive better signal almost all the time. Important point is: you get the same signal from different antennas in diversity system. And you select one of them or combination of all.

MIMO: MIMO system does have multiple antennas at the transmitter and receiver. Important point is, each of the transmitting antenna transmit different signals (data) and you receive different data at the receive antennas (you have to do some signal processing to extract different data transmitted from the transmitter). Therefore you increase throughput of the system.