From Google’s own Search blog:

Behind the simplicity of Google search is a complex set of algorithms that expands and improves the query you’ve typed to find the best results. Automatic spelling correction ([vynal] to “vinyl”) and substituting synonyms (matching [pictures] to “photos”) are just two examples of the improvements we make.

In most cases, Google’s algorithms make things better for our users – but in some rare cases, we don’t find what you were looking for…

Since then, we’ve received a lot of requests for a more deliberate way to tell Google to search using your exact terms. We’ve been listening, and starting today you’ll be able to do just that through verbatim search. With the verbatim tool on, we’ll use the literal words you entered without making normal improvements…

You can access the verbatim search tool under “More search tools” on the left-hand side.

via Search using your terms, verbatim – Inside Search.