Sunspots are temporary phenomena on the Sun's photosphere that appear as spots darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic field flux that inhibit convection. Sunspots usually appear in pairs of opposite magnetic polarity. Their number varies according to the approximately 11-year solar cycle.

The abundance of sunspots on the Sun varies on timescales from a few hours to many years. Historically, an index called the 'sunspot number' has been used to quantify the abundance of spots. This index is still in wide use today, although for some purposes it has been replaced by more readily and consistently measured indices such as the 10.7 centimetre solar flux. The main advantage of the sunspot number is that it is the only index for which we have a long and detailed historical record.

Sunspot Number (here denoted R) is defined as:

R = K * (10 * G + I)

where G is the number of sunspot groups visible on the Sun; I is the total number of individual spots visible; and K is an instrumental factor to take into account differences between observers and observatories.

Sunspot Number as an index can be defined on a daily basis but because of the large day-to-day variation is usually averaged over longer periods, the most common being the monthly and the yearly average. When averaged over a year, the sunspot number varies smoothly charting the progress of the solar cycle. On the other hand the daily and the monthly averages exhibit considerable variation with respect to the yearly curve. This variation is due to bursts of rapid solar region growth often associated with solar flares and other interesting events.

This cycle has been observed for centuries by changes in the Sun's appearance and by terrestrial phenomena such as auroras. Solar activity, driven both by the sunspot cycle and transient aperiodic processes govern the environment of the Solar System planets by creating space weather and impact space- and ground-based technologies as well as the Earth's atmosphere and also possibly climate fluctuations on scales of centuries and longer.

Accompanying the 11 year quasi-periodicity in sunspots, the large-scale dipolar (north-south) magnetic field component of the Sun also flips every 11 years; however, the peak in the dipolar field lags the peak in the sunspot number, with the former occurring at the minimum between two cycles. Levels of solar radiation and ejection of solar material, the number and size of sunspots, solar flares, and coronal loops all exhibit a synchronized fluctuation, from active to quiet to active again, with a period of 11 years