Theoretically you can't "uncrop" an image. That would entail pulling image data out of thin air. As far as I'm aware, there is no direct opposite term for "crop" - as in a term to mean "keep existing pixels, but add additional pixels around the outside of the image."
You can change the aspect ratio of an image from 3:2 to 4:3 or to 16:9 resulting in a distorted image. You can often "recrop" - change the cropping aspect ratio of an image - but you can't really "uncrop".
You can alter the canvas size a software application is using to display the existing image data, but that technically doesn't alter the image itself. At least not until it's saved. In which case, it then becomes a new image with a different size. Caveat: some software may remove or dump pixel (image) data upon resizing the existing canvas. So what is or is not lost by a resize of the canvas really depends upon the software in use.
Speaking strictly about Photoshop, it is possible to have image data which is hidden by being outside the Photoshop canvas. Choosing
Image > Reveal All will resize the canvas to show all existing image data. I guess that would sort of be an "uncrop". But the image data has to be there.