One "Tick" is 3 seconds. Tick is a player-invented term and is not referred to directly by in-game help text.
Regen, Refresh, Regain, and most Damage Over Time effects (except Helix Spells) all work in 3-second tick intervals. Every 3 seconds, the effects will process and your HP, MP, and/or TP "ticks" up or down. Thus, one "Tick" is 3 seconds.