An outline is a line wrapping up an element outside its borders. Its goal is to make the element prominent and capture users' attention. Unlike borders, the outline can take any size you want.

In CSS, we use the outline property and other various properties to adjust its style (e.g., dotted or solid), color, width, and offset. Outlines don't take up space — they go on the top of the container, overlapping with margin and nearby elements.