Every rainbow is circular, but from the ground we can see only a part of it

A rainbow is an optical and meteorological phenomenon that occurs when a bright light source illuminates a multitude of water droplets (such as rain or fog). It appears as a multicolored arc or circle composed of the colors of the visible spectrum (from red to violet).

If you look at a rainbow from a high mountain or an airplane, under the right conditions you can see the entire circle.