mainline

IPA: mˈeɪnɫaɪn

(aviation) An airline's main operating unit, as opposed to codeshares or regional subsidiaries.
(rail transport) The principal route or line of a railway.
(fishing) In longline fishing the central line to which the branch lines with baits are attached.
(plumbing) The pipeline carrying wastewater to the public drains or a septic tank.
(slang) A principal vein into which a drug can be injected.
(computing) The main repository for a software project, from which different versions (forks) may be split off.
(chess) Alternative form of main line [(chess) A standard sequence of opening moves considered to be best play.]

(slang) To inject (a drug) directly into a vein.
(by extension, informal, transitive) To consume voraciously.
(computing, transitive) To integrate (code, etc.) into the main repository for a software project, rather than separate forks.

Normal, principal or standard.
(rail transport) Of or pertaining to the principal route or line of a railway.
(rail transport) Of or pertaining to a surface railway as distinct from an underground, elevated or light rail one.
(chess) Of a sequence of opening moves: being part of a main line ("a standard sequence of opening moves considered to be best play").

main line

IPA: mˈeɪnɫˈaɪn

(chess) A standard sequence of opening moves considered to be best play.
Alternative form of mainline [(aviation) An airline's main operating unit, as opposed to codeshares or regional subsidiaries.]