1. (n.) A directress.

2. (n.) A line along which a point in another line moves, or which in any way governs the motion of the point and determines the position of the curve generated by it; the line along which the generatrix moves in generating a surface.

3. (n.) A straight line so situated with respect to a conic section that the distance of any point of the curve from it has a constant ratio to the distance of the same point from the focus.

aeronautics air lane air route air service airway astronautics aviation axis ballooning beeline blind flying chord cloud-seeding commercial aviation contact flying corridor cruising cut cutoff diagonal diameter direct line directrix edge flight flying general aviation gliding great-circle course lane normal path perpendicular pilotage radius radius vector right line sailing sailplaning secant segment shortcut shortest way side soaring straight straight course straight line straight stretch straightaway streamline tangent transversal vector winging