A microspan was a small unit of time used on Gallifrey. As employed by Lord President Romana, Darkel and K9 Marks I and II, it seemed to indicate a length of time somewhere between Earth seconds and minutes. (AUDIO: all of Gallifrey) Although Time Lords on Gallifrey used the term liberally, the Doctor never attempted to make his companions learn about the unit.
A unit of the same name was also used by Terpsivores, but it was unclear whether a "Terpsivorian microspan" was the same thing as a "Gallifreyan microspan". Twelve Terpsivorian microspans were less than two minutes. (COMIC: Death Disco)