From AMS Glossary
An area seen as a dark spot in contrast with the surrounding photosphere of the sun. They form because of concentrations of emergent magnetic flux that can be unipolar, bipolar, or complex groups with mixed polarities. They appear dark because they are cooler than the surrounding photosphere. Sunspots go through stages of development, often emerging as pores (small immature spots). As they mature, they can grow extensively in size, become more pronounced, and grow far greater in number, becoming a sunspot group. A sunspot's dark center is called the umbra, and more mature groups develop a surrounding area of less dark or lighter-shaded areas called the penumbra. They have lifetimes from hours to several months, and vary in occurrence and number with the solar cycle.
Space Weather Prediction Center, 2018: Sunspots/solar cycle. Accessed 1 October 2018. Available at https://www.swpc.noaa.gov/phenomena/sunspotssolar-cycle.
Term edited 1 October 2018.