Damage Over Time

Damage Over Time is a common Video Game mechanic where, instead of sustaining one-time sums of damage from one attack at a time, a unit receives a negative status that inflicts a small amount of damage at regular intervals, independent of any other factors or further attacks against them. The definition of an 'interval' varies by game: In action-based genres these intervals may be measured in real time, while turn-based genres (including non-video games, like a Tabletop RPG) may measure intervals based on "turns" or "rounds".

Depending on how long the effect lasts, the accumulated damage can become significant, especially if combined with an Area of Effect to harm multiple targets simultaneously. On a mechanical level, this is the opposite of Gradual Regeneration (and the "Regenerate" Status Buff), and some varieties may officially neutralize or counteract each other.

Note that characters who prefer defeating opponents via this method are almost always evil due to the connotations of slow, painful deaths (as opposed to the more-heroic quick and clean kills).

Damage over time can manifest from a wide variety of in-game sources:
 * Any number of Standard Status Effects -- "Poison" status is the obvious candidate here, but other status labels like "burn" or "bleeding" may cause it as well. A character who needs food badly may also lose HP over time from starvation.
 * A nearby hostile Field Power Effect
 * Falling into any Hazardous Water
 * Falling into Videogame Lava (or in some cases, just being in the same room as it, partially averting Convection, Schmonvection)
 * Characters who are Allergic to Evil (or vice versa)

(For sake of expediency, only list examples that are not covered by existing sub-tropes.)

Tabletop games

 * From Dungeons and Dragons
 * Module T1-4 The Temple of Elemental Evil. A PC in one of the four Nodes of Elemental Evil took 1-4 Hit Points of environmental damage per turn.
 * According to the Manual of the Planes (1987), the same thing happened on some of the Inner Planes.
 * Characters in the Elemental Plane of Earth took 1-2 Hit Points of damage per turn (from the pressure of the surrounding rock).
 * PCs on the Paraelemental Plane of Ice took 1-6 Hit Points of cold damage per round.
 * Fourth edition also features "Ongoing Damage", which is calculated at the start of each turn.
 * In Earthdawn, the nethermancer spell "Pain" inflicts minor damage to the target each round it's in effect.
 * The Sixth Edition Hero System has a "Damage Over Time" advantage that can be added to most powers and attacks.
 * Magic the Gathering has several cards that deal damage during a player's "Upkeep" step, in contrast to most cards which can only deal damage once at a time.

Video Games

 * In Borderlands, many weapons that deal elemental damage have a chance of causing damage over time as an added effect. Incendiary and corrosive DOT is relatively mild but lasts for a long time, while shock DOT wears off more quickly, but is much harsher.
 * In Civilization 2, helicopters received minor damage for every turn they spent in midair -- this was intended to simulate their limited fuel reserves without requiring them to return to base every time. Later Civ games removed this.
 * In the first Dark Cloud, if a character's "Thirst" meter runs dry during a crawl, their HP begins to drain. (In the sequel, Thirst was merely a Standard Status Effect that blocked healing.)
 * In Earthbound, whenever a character receives damage or healing, their Life Meter rolls down or up to the new value over time (rather than instantly), the speed of which is governed by the character's individual "Guts" stat. Side effects like Critical Existence Failure do not trigger based on the raw damage a character has received, but the value that's currently shown on their meter instead.
 * In the original Gauntlet, the players' HP decreased at a constant rate throughout the game. The only means to replenish HP? Food items in various levels, or putting more coins in the machine.
 * In I Miss the Sunrise, during certain boss missions, you can place your secondary fleets on certain tiles to gain "fire support" from them, causing a small but reliable amount of damage to the boss per turn.
 * Kingdom of Loathing has various items, pieces of equipment and statuses that inflict "passive damage" over time.
 * Odin Sphere, in addition to its set of Standard Status Effects, has fire and ice levels which sap 1% of your character's HP at regular intervals unless you drink an appropriate curing potion beforehand (or have elemental protection equipped).
 * The Pokémon series has many ways to inflict Damage Over Time beyond its standard "Poison" and "Burn" statuses, and many of these can even be combined:
 * If a Ghost-type uses "Curse", the opponent receives significant damage (1/4 max HP) per turn, the largest amount of damage in the system.
 * Hazardous weather like "Sandstorm" or "Hail" inflicts 1/16th damage on most elemental types.
 * Certain abilities can also cause damage (or, inversely, healing) over time during specific weather conditions: "Dry Skin" damages during intense sunlight, "Rain Dish" and "Ice Body" heal during heavy rain or hailstorms, respectively. "Solar Power" also causes damage during intense sunlight, but with the tradeoff of boosted attack power.
 * In Pokémon XD: Gale of Darkness, Shadow Pokemon who enter "Reverse Mode" in battle sustain damage per turn, as do non-shadow Pokemon when subjected to "Shadow Sky" weather.
 * Sleeping Pokemon receive damage while subjected to "Nightmare" status, or similarly, the "Bad Dreams" effect of legendary Pokemon Darkrai.
 * The Grass-type "Leech Seed" not only inflicts Damage Over Time on an opponent, it also restores the user's HP by the amount drained.
 * The "Sticky Barb" item inflicts damage-over-time on whichever Pokemon holds it, but can be passed from user to opponent by moves involving physical contact.
 * In Prince of Persia: Warrior Within and The Two Thrones, the player slowly loses health while playing as the Sand Wraith or the Dark Prince.
 * The hallmark ability of the Pyro class in Team Fortress 2 is the ability to set opponents on fire with their flamethrower. Some melee weapons can inflict a "bleeding" status that also causes damage over time.
 * World of Warcraft:
 * The "Warlock" class is primarily built around skills that cause damage over time, especially if specialized in Affliction.
 * Feral Cat Druids deal damage primarily through Rake and Rip, which make the target bleed in a similar way to a Warlock DoT. They're the hardest DPS spec to master, but deal insane damage when done right.
 * Dungeons of Dredmor includes a variety of effects that cause damage over time, including but not limited to Slimeburst Traps, Acid Burns, Curse of the Golden Ratio, Recursive Curse, and the special ability of the Golden Crossbow. There's also an even wider variety of Area of Effect spells that persist over several turns and deal constant damage to anything in range--these ones tend to be very effective for dispatching large groups of enemies.
 * In Xenoblade Chronicles, any arts of the fire, ice, or earth elements will leave behind a burn, chill, or poison debuff on the victim that causes gradual damage. Some physical arts also leave a bleeding effect. In all cases, the gradual damage dealt is based on how much the initial attack hit for, which can result in some crazy numbers once damage multipliers from gems, skills, and chain attacks come into play.