The term "diversity" is often used when discussing wireless mics.
Diversity refers to the use of multiple (usually two) antennae with a wireless receiver so that the signal from the transmitter (microphone or beltpack) will have multiple paths to the receiver.
Since wireless mic signals travel through the air to the receiver through both direct and reflected paths, having multiple antennae improves the probability that the transmitter's signal will be received without interference.
There are two types of diversity.
The first, passive diversity, simply means that a second antenna is installed on the receiver. The addition of the second antenna lets the receiver gather more of the transmitted radio signal, but in reality, this does little to limit signal drop out.
Active switching diversity lets the signals from two antennae be mixed together into one input. Because the receiver is able to "switch the phase" of the signal of one of the antennae, the likelihood of drop outs is decreased.
Active switching diversity receivers are most often used in high end UHF systems.