A plane is a surface such that if any two points are taken on it, the line segment joining them lies completely on the surface. It means every point on the line segment joining any two points on the plane lies on the plane.
Every first degree equation in x,y, and z represents a plane.
ax +by+cz+d = 0 is the general equation of the plane.
We can also write it as
Ax + By+Cz+1 = 0; This is an equation in three unknowns.