WHAT DOES A PERPENDICULAR MEAN?

A Perpendicular is any line which is stands at a right angle to the given line.
The 'perpendicularity' can be with planes or lines.
When one speaks about lines, two lines could be declared as perpendicular in two ways:

• One when the angle between them is 90 degree and so is the congruent angle too.
• Other, when the lines satisfy the condition of the perpendicularity with respect to slope.

In a graph, the condition of 2 lines being perpendicular can be observed in 3 cases:

• One when the line is having an x coordinate value = 0 and some value for the y coordinate ( a line which would be parallel to the x-axis)
• Another when the line is having an y coordinate value = 0 and some value for the x coordinate ( a line which would be parallel to the y-axis)
• Another when the product of the slopes of both the lines result in -1.(minus 1) 

This way under graphical drawings, two lines can be declared perpendicular.