Damage-over-time (also referred as DoT for short) is a type of damage that involves damaging bloons over time by a harmful status effect. The rate of creating an arbitrary amount of damage over a specific time is referred as a "damage tick". Duration of a DoT will determine how many ticks that such damage-over-time will do over its lifetime while not refreshed. Most types of DoT can be refreshed by making an attack that inflicts the same type of DoT. Bloons can have many unique types of DoT at once, but some types (e.g. different Burn 
or different Acid 
) cannot have the same type stacked. In those cases, there may be special rules with mixing DoTs of the same type, such as purple acid always overriding red acid.
Stackable damage-over-time effects were formerly a feature in Bloons TD 6, but due to memory leaks (i.e. malfunctioning allocation of RAM, thereby causing a decay in game performance) it was removed from both Glaive Lord's Shred 
in Version 27.0 and Ezili's Curse 
in Version 28.0. While this was intended behavior for Glaive Lord's on-hit shredding DoT, this was not intended for Ezili, and she likely lost that unique stackable DoT as a result of both removing an unintended feature and to remove the performance-deterioration that stackable damage-over-time causes.
Damage-over-time's damage stacks with all damage buffs, with exception of Alch buffs or Jar of Pickles's buffs. This is noticeable with all debuff effect e.g; 2-4-0 Glue Strike ability's glue, as well as Striker Jones's Level 20 Artillery Command when used with higher-tier burning from Mortars.
