1. A computer program product for a search engine, the computer program product comprising: one or more computer-readable storage media and program instructions of the search engine stored on the one or more computer-readable storage media, the program instructions comprising: program instructions to receive a search expression; program instructions to extract, using a search term extractor of the search engine, two or more query terms of the search expression; program instructions to determine 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; program instructions to determine 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; program instructions to determine 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; program instructions to select 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; program instructions to generate an expanded search expression by expanding the search expression using an associated text of the at least one node; and program instructions to generate, by the search engine, an output of search results by executing a search using the expanded search expression.