At least three copies of data, stored on two different types of media, with one copy kept off-site.
The production version counts as the first copy, while a local backup—such as one stored on an external drive or network-attached storage device—provides the second. Using different media reduces the chance that one hardware failure will affect every copy.
The third copy is stored in a separate physical location, often in cloud storage. This protects the data against site-wide events such as fire, theft, flooding, or other disasters.