A line that intersects a given set of two or more lines in the plane. When a transversal intersects a given pair of lines, eight angles are formed; the four angles between the pair of lines are interior angles and the four others are exterior angles.

Corresponding and alternate angles

The figure shows a transversal intersecting two lines. The angles α‎ and β‎ are called corresponding angles and α‎ and γ‎ are called alternate angles. The given two lines are parallel if and only if corresponding angles are equal: they are also parallel if and only if alternate angles are equal.

The term ‘transversal’ also relates to a line intersecting to other lines in higher-dimensional spaces.