Pixel density refers to how many pixels fit into one inch of a screen, measured in PPI (pixels per inch) or DPI (dots per inch). Higher pixel density means sharper, clearer images and text. Modern smartphones typically have pixel densities ranging from 300 to 500 PPI, while tablets might have slightly lower densities due to their larger screens.

Device manufacturers often use marketing terms for their high-density displays. Apple calls their high-density screens "Retina Display," which started at 326 PPI with the iPhone 4. Other manufacturers use terms like "Super AMOLED" or "Liquid Retina," though these terms also describe other screen qualities such as color accuracy, brightness, contrast ratio, and screen technology.

Understanding pixel density helps designers create sharp graphics and readable text. For instance, text that's easily readable on a low-density display might appear tiny on a high-density screen if not properly scaled. This is why designers work with relative units rather than fixed pixel sizes.