When you put a tilde ~ in front of your search term, Google will look for both your chosen search term and synonyms. Do not put a space between the tilde and your search term. For instance, searching for ~ranch yields results relating to cattle, horses, and farms. The synonyms are bold in the search results, just as if you'd searched for ranch OR horse OR cattle OR farms.
The More the Merrier
You may search for as many at once as you wish. Just put a tilde in front of every term you'd like to include. Google suggests, for instance, searching for ~food ~facts for information on recipes and nutrition.
Your Results May Vary
Google's synonyms are chosen by machines, not a thesaurus, so some of the words it thinks are synonyms may not sound like synonyms on the surface. For instance, on of the synonyms in the ~ranch search was "cattle" and "resort."