A dial is a circle that has a series of numbers or marks representing numbers, like the dial of a clock face, a compass, or an old-fashioned telephone. If you have a watch that glows in the dark, it's the dial you can see even in a dark movie theater.
Old telephones and televisions both have dials — the first has numbers from zero to nine in a circle, and the latter has a round disc for changing channels. When you call someone, you can say that you dial their number. In fact, people tend to use the word dial when they use a phone, even if they're pushing buttons or tapping a touch screen. The Latin root is dialis, "daily," and the earliest dial was a sundial.