Pronunciation: (woolf; Fr. vôlf), [key] Astron.
a number indicating the degree of sunspot activity on the sun as a factor of observer idiosyncrasies, the number of sunspot groups, and the number of individual sunspots. Also called relative sunspot number, sunspot number.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.