Gas lift:

The process of raising or lifting fluid from a well by injecting gas down the well through tubing or through the tubing-casing annulus.

Injected gas aerates the fluid to make it exert less pressure than the formation does; consequently, the higher formation pressure forces the fluid out of the wellbore.

Gas may be injected continuously or intermittently, depending on the producing characteristics of the well and the arrangement of the gas-lift equipment.