What is a "flight corridor"?

Imaginary tubes in the sky (they have floors, ceilings and edges). Designed by aviation authorities to simplify air traffic control. They appear on aviation maps as roads between airports, navigation aids and with the advent of GPS, points in space. There are many different kinds but the most common is the Victor airway mentioned by someone else. However the term also applies to corridors designed for military operations to regulate military traffic over a theater of operations (the Balkans, Iraq, Afghanistan). Those are much more complex and ridgid.