1. A method of a search engine comprising: receiving, by one or more processors, a search expression; extracting, by one or more processors, two or more query terms of the search expression using a search term extractor of the search engine; determining, by one or more processors, two or more nodes representing a first term of the two or more query terms and at least one node representing a second term of the two or more query terms, wherein the two or more nodes each have an associated text for search term expansion and represent at least one concept in a semantic graph of nodes that represents a domain of semantically related concepts; determining, by the one or more processors, a center of focus within the semantic graph for the two or more nodes based, at least in part, on a spreading activation in the semantic graph; determining, by the one or more processors, a contextual relevance for the two or more nodes with respect to the center of focus based, at least in part, on an assessment of semantic similarity for the two or more nodes with respect to the center of focus; selecting, by the one or more processors, for a query term, which is included in the two or more query terms of the search expression, at least one node from the two or more nodes based, at least in part, on a contextual relevance between the at least one node and the determined center of focus; generating, by the one or more processors, an expanded search expression by expanding the search expression using an associated text of the at least one node; and generating, by the one or more processors, an output of search results by the search engine executing a search using the expanded search expression.
4. The method according to claim 1 , the method comprising: configuring, by the one or more processors, the spreading activation such that a balance between over-connected and under-connected nodes is generated.