Image Signal Processor
Hardware devoted to processing image data. In a phone, the ISP typically handles data that comes from the camera sensor(s), turning that into usable photos and/or videos.
The ISP may also apply various kinds of processing, such as improving image quality, particularly in low-light or high-contrast scenes.
The ISP is typically just one part of the main chip that powers the phone, which is called an SoC (system-on-chip).