Concept Searching allows you to retrieve documents based on concepts.
For example, a search for "GPS" would also automatically retrieve documents containing the phrase "Global Positioning System". This works both ways, a search for "Global Positioning System" would also find documents containing only "GPS".
The concept search can be a powerful tool in that the concept itself can be a boolean expression. E.g., Intel used the name of a River as the development name for it's 64 bit microprocessor chip, Merced. You could define a concept, "Merced" so that any time you searched for Merced, you are really searching for "Merced and not Merced River".