A type of intermittent multiphase fluid flow regime that is characterized by pockets of gas in the form of high-velocity gas bubbles. In vertical flow, the gas is in the form of axially symmetrical bullets known as slugs that occupy most of the cross-sectional area of the pipe. In horizontal flow, the gas is also in the form of pockets of gas, but they are not symmetrical although they do have a curved nose in the direction of travel. In both cases, the resulting flow alternates between high-liquid and high-gas composition. Slug catchers at the end of pipelines are used to disengage the gas. Semi-slug flow occurs where the surges do not completely fill the pipe and is often considered to be a form of wavy flow.