Cooldown is a measurement of time between unit's attacks.
In StarCraft, the cooldown is measured in frames per second, which is 16 frames per second on Normal, 24 frames on Fastest. Cooldown also randomly changes by -1 to +2 frames between each attack. The smallest possible cooldown in Starcraft is 5, and the biggest possible cooldown is 250.
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 by increasing their cooldown by 50% (rounded down).
Some units' cooldowns are primarily controlled by their iscript, like Ultralisks. For these units, Ensnare and Acid Spores have less noticeable effects.
In StarCraft II, the cooldown is measured in seconds between attacks, as in Warcraft III. Up until the release of Wings of Liberty, weapon tooltips instead used words such as "fast" instead of numbers.