“Google introduced a new advanced search feature that enables users to search not only for a particular keyword, but also for its synonyms. This is accomplished by placing a ~ character directly in front of the keyword in the search box.
For example, to search for browser help as well as browser guides and tutorials users can search for browser ~help. The ~ character was chosen because it’s shorthand for approximate and a good way for users to express their wish to expand searches to include synonyms. More information about this feature is available here.” [via GoogleWeblog, and attributed to Nate Tyler, who is Public Relations Manager at Google Inc.]