Google was sued on Tuesday in a federal court for allegedly deceiving an advertiser and charging for adverts displayed on third-party websites, which the advertiser never asked.

The case is surrounding Google's AdSense program which targets ads to keywords in articles and other content at particular sites, complementing the AdWords program which targets adverts alongside Google's search results.

According to the lawsuit, Google is accused of defrauding advertisers out of millions of dollars by collectively redefining the universally understood meaning of an input form left blank.

The advertiser, David Almeida, had signed up for Google ads to promote his private investigation business in Massachusetts but he didn't want to buy AdSense ads.

Almeida said he left the maximum per-click bid blank thinking that the word optional meant he could opt out of the AdSense program by doing so but the AdWords bid replied when he did not exercise that option.