Latent Semantic Indexing. All of the Search Engines have invested in this technology and if they are not using it now, they will be soon.
The definition for LSI is as follows:
Latent semantic analysis (LSA) is a technique in natural language processing, in particular in vectorial semantics, patented in 1988 [1] by Scott Deerwester, Susan Dumais, George Furnas, Richard Harshman, Thomas Landauer, Karen Lochbaum and Lynn Streeter. In the context of its application to information retrieval, it is sometimes called latent semantic indexing (LSI) – http://www.cs.utk.edu/~lsi/
