I have to know about this. I have found this term when I search on the web. Even I have read this topic in Wikipedia. I did not understand the concept.
I think you need to separate these two words. Talking about semantic which is an adjective. The noun form 'semantics' is a branch of study of languages in relation to meanings. This term has of late been in uses in a highly technical sense in the field of software. As to search I don't think it's the right word in this context. It could be research.
If the search engine searches keywords not on based meanings of it instead it searches regarding to the context of content in which keywords are used. I have understand this much. I would like to know some more about semantic search.
