1. A computer-implemented method comprising: under control of one or more computer systems configured with executable instructions: retrieving at least one activity record that represents at least one interaction between a visitor and a server-based system; identifying within the activity record at least one input provided by the visitor during the interaction; identifying within the activity record at least one item associated with the input; accessing at least one further activity record containing the input; comparing the activity record and the further activity record; identifying within the further activity record at least one further item associated with the input; establishing a similarity rating of the item and the further item based at least in part on the comparison of the activity record and the further activity record; determining, based at least in part on the similarity rating of the item and the further item, that a document associated with the item and a further document associated with the further item contain synonyms; designating a term that appears in the document associated with the item as a synonym for a different term that appears in the further document associated with the further item; receiving a search query specifying at least the different term that appears in the further document; and returning, as search results for the search query, the further document associated with the further item and the document associated with the item containing the term that is the synonym for the different term.