What is an orthogonal projection to a plane?

Answer 1

A project is a linear function P that satisfies P(x)=P(P(x)) for all x that is well defined.

An orthogonal projection is a projection that gives the point its coordinates with regard to the orthogonal coordinate that it is on.

An orthogonal projection to a plane is an orthogonal projection that the co-domain must be a subspace of the plane.

Answer 2

see the link:

http://en.wikipedia.org/wiki/Projection_operator