What does perpendicular mean mathematicaly?

Answer 1

In elementary geometry, the property of being perpendicular (perpendicularity) is the relationship between two lines which meet at a right angle (90 degrees). The property extends to other related geometric objects. A line is said to be perpendicular to another line if the two lines intersect at a right angle

Answer 2

Perpendicular is mostly used to describe Shapes and their properties. It means when two lines meet up at a 90 degrees right angle. It might be an X or a T when you look at it. Like the example shown below:

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So that isn't a good drawing, but it does show you the meaning of perpendicular!