Damage Over Time, often abbreviated to DOT or DoT, is the concept of steady consistent damage, rather than a single large strike.
There are a wide range of DoT spells within Vana'diel, usually dealing small amounts of damage, over a long period of time.
This damage can add up considerably over time.
Some DoT spells also impart a Debuff effect upon their target alongside the DoT effect.
Some melee jobs are categorized amongst the community as DoT jobs, as opposed to DD/'spike' jobs.