Damage Over Time
Damage Over Time, often abbreviated DoT, refers to any of several detrimental status effects or the spells/abilities which inflict them, where damage is done to the affected target in small amounts at set time intervals (often referred to as ticks) for a certain duration. They are often highly desirable on enemy targets with high HP to gradually damage them, as these abilities generally offer a high amount of damage per activation and/or resource cost compared to abilities that deal all of their damage at once. The latter offer higher damage per second but less total damage over a given duration and/or less damage for a given use of that ability. DoT effects are much less useful against enemies that can be defeated in a short period of time (significantly shorter than the duration of DoT effect), as the per-activation potential of the DoT ability will not be realized. Optimally, DoT effects should be applied early in a fight and re-applied only when they are very close to expiring, as the earlier effect will be immediately replaced with the new one.
That said, players need to be careful of DoT effects, such as Poison, both in their use and avoidance of being affected by them. DoT effects can K.O. a player not watching their HP level, and from a healer's perspective, such effects represent unnecessary damage that will have to be healed but which could be prevented with early use of an Esuna spell. A DoT effect on an enemy or player will also normally prevent them from being put to Sleep.
Damage Over Time abilities benefit from the Spell Speed and Skill Speed gear properties by dealing more damage per tick in addition to merely being faster to recast, which would otherwise be of negligible benefit.