The intersecting lines which contain right angle between them are called perpendicular lines.
Straight lines are often intersected in a plane in geometrical system. Occasionally, they get intersected by an angle of $90$ degrees (right angle).
In this case, the straight lines are called as perpendicular lines.
$\small \overleftrightarrow{AB}$ and $\small \overleftrightarrow{CD}$ are two straight lines and they are intersected by an angle of $90^\circ$ in a plane at point $\small O$. Therefore, the straight lines $\small \overleftrightarrow{AB}$ and $\small \overleftrightarrow{CD}$ are said to be perpendicular to each other.
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