The term 'PHONE ADD-ON' can be summarized as a device or accessory that can be attached to a phone to enhance its capabilities. The crossword solution for this term is 'MEGA, ' which is short for megapixel, a unit of measurement used to describe the resolution of digital cameras and other imaging devices. Megapixels are used to measure the number of pixels in an image, and higher megapixel counts generally result in higher-quality images. Therefore, a phone add-on with a high megapixel count would likely improve the quality of photos taken with the phone.