A "tick" is one set interval for an "Over-time" ability or effect to take place.
The standard tick is exactly three seconds. This is used to trigger the effects of DOT spells such as Poison and Choke as well as Regen-line spells and effects, including Refresh and the TP effect of Berserker's Torque.
Another type is the "Healing Tick", which is 10 seconds. This is the time between each MP and HP gain while resting (See MP Recovered While Healing). Note that the first Healing Tick restores 0 HP and MP.