A "tick" is one set interval for an "Over-time" ability or effect to take place.
The standard tick occurs every 3 seconds (Earth time). This is used to trigger the effects of DOT spells such as Poison and Choke as well as recovery effects such as Regen, Refresh, and Regain. Such effects generally last for a certain predetermined amount of ticks before wearing off.
Another type is the "Healing Tick", which occurs every 10 seconds (Earth time). This is the time between each MP and HP gain while resting with the /heal command. Note that the first Healing Tick occurs after 20 seconds instead of 10 seconds.