The existing ambiguity of the notion of software engineering is mainly due to the fact that it is based on and depends on knowledge. The new definition of the term "software engineering", proposed in this paper, encounters that fact. The main subject of discussion in the paper is how three different types of knowledge, namely declarative explicit, declarative structured (ontologies) and tacit can be used for effective support of software engineering as both practice and academic subject. Illustrative examples are shown along with some trends for more intensive use of knowledge for support of software engineering.