Cooldown is a measurement of time between a unit's attacks.
In StarCraft, the cooldown is measured in frames per second, which is 15 frames per second on Normal, 24 frames on Fastest.
Special abilities do not necessarily have a cooldown time. Several units have upgrades or abilities that change their cooldown time, such as the stimpack ability for marines and firebats, or the Adrenal Glands upgrade for zerglings. Devourers are dangerous air-to-air units as they inflict a delay of 1 and increase damage taken by 1. The queen's Ensnare ability also slows down enemy attacks.