Hello, zak100!

Your wording is confusing . . .

Can i use the term "below the plane" for a point inside the plane,

i.e when Ax + By+ Cz + D < 0?

The book uses the term "inside the plane" and the web material which i have searched.

However still some people use the phrase "below the plane".

Is it same as "inside the plane"?

A plane is defined by an: .equation

Any point that satisfies the equation

. . is said to bethe plane.on

With an, likeinequality

. . we have all the points which arethe plane.below

Your book should not use the term "inside".

There is a great difference between "on the circle"

. . and "inside the circle".

Besides, "inside" means "within its boundaries".

A plane has no boundaries ... it extends infinitely.