Scientific Socialism is the term used by Friedrich Engels to describe the socio-political-economic theory pioneered by Karl Marx. The reason why this socialism is "scientific socialism" (as opposed to "utopian socialism") is because, like science, observation is essential in this theory. Although Marx denounced "utopian socialism", he never referred to his own ideas as "scientific socialism".
There is an emphasis on materialism, as opposed to religion or idealism. Marx and Engels observed history and came to the conclusion that history is shaped by materialism. This is called historical materialism, a fundamental theory within the theory.