Scott Suiter, a former Google intern, asked 50 passersby from New York what is a browser and the responses were almost unanimous: a browser is a search engine. Some even said that their browser is Google.
According to Wikipedia, "a web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web." But search engines are the applications that help you find web pages and they're so important that they became synonymous with web browsers. It's an involuntary synecdoche, a figure of speech in which a part of an object is used to refer to the entire object.
{ via Andy Baio }