parallel lines
Two lines that point in exactly the same direction and never meet, however far they're extended.
Two lines are parallel when they point in the same direction and never intersect, no matter how far extended. Affine transforms always keep parallel lines parallel; projective transforms do not, since they can bend originally parallel lines toward a shared vanishing point.
Examples
The two rails of a train track are parallel in the real world, but a photo taken from ground level shows them appearing to meet near the horizon.
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