What is the term for using the pixels in one layer to hide another layer?

The term that defines using the pixels in one layer to hide another layer is known as a Track Matte. When you use a Track Matte, the transparency of one layer is dictated by the pixels of a different layer. For example, if you have a text layer and an image layer, you could use the text layer as a Track Matte for the image. This means that wherever the text appears, the image will be visible, and where the text does not appear, the image will be hidden.

This technique is commonly used in motion graphics and compositing to create dynamic effects, allowing more complex visual combinations than simply changing opacity or using a layer mask alone. In contrast, while Alpha Channel refers to the transparency information in an image file, Layer Masks allow for non-destructive masking on the same layer, and Opacity Adjustment involves changing the transparency of a layer uniformly rather than through control of another layer's pixels.