A span was a unit used to measure distance.
It was intended that Eldrad's obliteration module be detonated twenty-five spans out from Kastria, but fears that the loss of power might jeopardise control over the craft led King Rokon to authorise its use at only nineteen spans. This allowed Eldrad to survive. (TV: The Hand of Fear)
When a foreign spacecraft entered a two span radius of Gallifrey, Rodan put out a red alert and ordered that the transduction barrier be raised to factor five. (TV: The Invasion of Time)
Euterpian ships had an artificial day which lasted 15 spans, while the Earth's daily rotation was 11 spans. (PROSE: Invasion of the Cat-People)