A deepfake is an image or video where the original person's likeness is replaced by another person's. It's usually done with celebrities in movie clips. In the example below, Amy Adam's face (left) is replaced with Nick Cage's (right). This is done using machine learning; many images of both people are uploaded to a computer and over time, the computer can learn how to match one to the other. Deepfakes started appearing around the end of 2017, but as access to the technology grows, the potential to use it maliciously grows too.
Click the image for copyright info.
For more on how deep fakes work and why they could be a cause for concern: