Spatial diversity is any one of several wireless diversity schemes that uses two or more antennas to improve the quality and reliability of a wireless link. Often, especially in urban and indoor environments, there is no clear line-of-sight (LOS) between transmitter and receiver. Instead the signal is reflected along multiple paths before finally being received. Each of these bounces can introduce phase shifts, time delays, attenuation, and distortions that can destructively interfere with one another at the aperture of the receiving antenna.
Antenna diversity is especially effective at mitigating these multi-path situations. This is because multiple antennas offer a receiver several observations of the same signal. Each antenna will experience a different interference environment. Thus, if one antenna is experiencing a deep fade, it is likely that another has a sufficient signal. Collectively such a system can provide a robust link. While this is primarily seen in receiving systems (diversity reception), the analog has also proven valuable for transmitting systems (transmit diversity) as well.
An antenna diversity scheme requires additional hardware and integration versus a single antenna system but due to the commonality of the signal paths a fair amount of circuitry can be shared. Also, multiple signals demand greater processing from the receiver, which can lead to tighter design requirements. Signal reliability is paramount and using multiple antennas is an effective way to decrease the number of drop-outs and lost connections. 
Your Dictionary: Spatial Diversity