You mean "orthogonal projection"? If we suppose "a" is a nonzero vector in and "x" happens to be any vector in , then the orthogonal projection of x onto span{a} is written as and is defined using this formula:

The projection vector can be called "vector component of x along a".

Edit: If you consider the operator that "projects" any vector onto a line through the origin, by dropping a perpenicular to that line, it's called an orthogonal projection. This is the definition of projections onto lines through the origin of .