Resolution
The measure of an image's fixed detail, usually expressed as its PPI or the DPI of its output device.
In practice
In this dictionary, resolution means how much fixed detail an image holds, usually stated as pixels per inch for the image or dots per inch for the device that outputs it. Low resolution shows up as blur or visible pixels when an image is enlarged. A logo copied from a website may look fine on screen but print fuzzy on a business card. Screen makers also use the word for a display's pixel dimensions, such as 1920 by 1080.
Often confused with PPI
Resolution is the general idea of fixed image detail; PPI is one specific way of measuring it, as pixel density at a given size.
Try this
Print a small web image at business-card size and at letter size, and note where it starts to look blurry.