A measurement of pixel density. It represents the number of PIXELS fit across and Inch when an image is printed. The standard number of pixels for a clear print image is 300 pixels per inch. For an image on a screen this number is 72 pixels per inch.
A feature of Photoshop that allows you to combine multiple photos into a single image by "masking" or hiding certain areas of that image. Note the example below. The Logan layer has the original image but it is masked.