The term "mainline" can have different meanings depending on its context. Generally, it refers to something that is considered traditional or conventional within a particular group or culture. In this sense, mainline could describe an established and accepted practice, belief, or behavior that is widely shared by the majority of people in a community.
In another usage, "mainline" can refer to a drug taken intravenously directly into a person's bloodstream through a vein. This type of administration allows for rapid entry into the bloodstream and quick onset of effects.
The term may also be used to describe an aircraft that is designed to take off and land horizontally, typically from an airbase or carrier. These planes are often larger than other types of aircraft and can carry a significant amount of weapons or cargo.