The term 'DARK PART OF A SUNSPOT' can be summarized as the 'UMBRA, ' which is a five-letter word. The UMBRA is the darkest part of a sunspot, which is an area of reduced surface temperature on the Sun's photosphere. Sunspots are caused by intense magnetic activity and appear as dark spots on the Sun's surface. The UMBRA is the innermost part of the sunspot, where the magnetic field is strongest and the temperature is lowest.