What is cross polarised antenna?

Its an antenna that can operate in vertical as well as horizontal polarization. Dual polarized antennas are often used in 802.11n devices to enable multi-thread communication. This enables the device to transmit data on one polarization and receive data on the other simultaniously. The advantage of this is that multi-thread communication can be achieved with the use of one antenna instead of using two independent (and opposite polarized) antennas for the same result. Obviously all the networking equipment in the wireless network must be multi-thread complaint for this to work.