Fingerprinting collects information about a person's device and browser and puts it together to create a profile of that person.
The information is not explicitly collected in order to advertise to people, but it can be used to target specific ads based on that user's data.
For example, a person's screen size or language settings are legitimately needed in order to display a website properly.
But when that is combined with their time zone, browser type, battery level - and many other data points - it can create a unique combination of settings which makes it easier to work out who is using a web service.
These details along with someone's IP address - the unique identifier used by internet devices - were previously prohibited by Google for ad targeting.
Privacy campaigners say that unlike cookies, which are small files stored on a local device, users have little control over whether they send fingerprinting information to advertisers.
"By explicitly allowing a tracking technique that they previously described as incompatible with user control, Google highlights its ongoing prioritisation of profits over privacy," said Lena Cohen, staff technologist at the Electronic Frontier Foundation.
"The same tracking techniques that Google claims are essential for online advertising also expose individuals' sensitive information to data brokers, surveillance companies, and law enforcement," she added.
