The description of the word you requested from the astronomical dictionary is given below.
Sunspots are regions on the solar surface where the magnetic field is very strong, up to 3000 gauss in strength. Sunspots look a lot darker than their surroundings in almost all kinds of observations, because they are a few thousand degrees cooler than their surroundings and because they are also large. Sunspots range in diameter from about 2500 to over 50,000 km. A sunspot is often roughly circular in shape, though some have a very irregular appearance. Sunspots have two very distinct parts: the dark umbra in the middle, and the less-dark penumbra around the umbra. The difference between a sunspot and a pore is that a pore has no penumbra.