Given points A,B, the line joining them, and a third point in space C, let C' be the closest point to C on AB. There are many names and contexts for this relationship. C' is the projection of C on the line AB. In triangle ΔABC, CC' is the altitude of vertex C on base AB.
In 3 dimensional space, -The dot product remains a scalar (so that's what I'll use). -There will always be only one vector v which is both perpendicular to u and lies in the same plane.