From Latin: perpendicularis "vertical, as a plumb line"

A line is perpendicular to another if it meets or crosses it at right angles (90°).

Try this Drag the orange dot at point A. The line AB will be perpendicular to DF only when it meets it at 90°

Perpendicular means "at right angles". A line meeting another at a right angle, or 90° is said to be perpendicular to it.
In the figure above, the line AB is perpendicular to the line DF. If they met at some other angle we would say that AB meets DF 'obliquely'.
Move the point A around and create both situations. Move the mouse carefully to get AB exactly perpendicular to DF.

Symbols

AB ⊥ DF

is read as "AB is perpendicular to DF"

Normal

Another term used for the same concept is 'normal'. When line A meets line B at 90°, we say that "A is normal to B".
So in the diagram above, after pressing 'reset',
AB is normal to DF.

How to draw a perpendicular

It is possible to draw a perpendicular to a line without any measurement using just a compass and straightedge using techniques developed thousands of years ago by the Greeks.
For an animated demonstration of how to do this see