Semantics is the study of meaning. So a semantic search is one which “seeks to improve search accuracy by understanding searcher intent and the contextual meaning of terms … to generate more relevant results.” (http://en.wikipedia.org/wiki/Semantic_search) Human knowledge and the language that humans use to communicate that knowledge are ambiguous and messy.
Take the term” diamond,” for example. It could refer to jewelry or sports. If I am searching for “mercury,” is that the planet, the element, or the car? We see the problem in business every day. People the same term to describe different things, and they also use different terms to describe the same thing. They use vague terms such as “searching for a solution” or “looking for an agreement.”
Most organizations have problems with search not because the search engine is inherently flawed. Keyword search engines do what they are told: they look for word occurrences. Humans and human language is the bigger problem. For those organizations mature and disciplined enough to utilize the tools and technologies currently available (such as metadata management in SharePoint) it is possible to create very effective search experiences.
Source: Earley & Associates