In category theory, two morphisms $e:A\to B$ and $m:C\to D$ are said to be orthogonal if for any $f:A\to C$ and $g:B\to D$ with $m\circ f=g\circ e$, there exists a unique morphism $d:B\to C$ such that $f=d\circ e$ and $g=m\circ d$.
What a possible interpretation of this concept? And why is it called "orthogonal", does this help the intuition in some way? Is there a standard example where the name becomes clear?
An introductory reference would also be welcome.