Chi Sao (黐手 literally, "sticky hand") is a Wing Chun term for the principle, and drills based on the principle, of "sticking to" an opponent.
In Wing Chun this is practiced through two practitioners maintaining contact with each other's forearms while executing techniques, thereby training each other to sense changes in body mechanics, pressure, momentum and "feel". This increased sensitivity gained from this drill helps a practitioner attack and counter an opponent's movements precisely, quickly and with the appropriate technique.
Chi sao is very similar to the hubud-lubad drills of Eskrima, and looks somewhat like the push hands training of T'ai Chi Ch'uan. Chi sao is also taught in the Jeet Kune Do? traditions, and uses modified versions of some of the component techniques such as the bong sao and jut sao.
Baguazhang uses its own form of chi sao, which involves one hand of each practioner being "stuck together" at the wrist while they try to unbalance each other or perform a Chin Na(joint-lock) technique.