Cumulative upkeep is a keyword ability on permanents that requires the permanent's controller to pay an increasing cost during each of his or her turns to keep the permanent in play.
Cyclone from Arabian Nights is the first card printed to use cumulative upkeep, although it did not use this keyword. (Stasis is considered the first card with cumulative upkeep by some, but its upkeep cost is only similar to, and not descriptive of, cumulative upkeep, and the card's Oracle text does not use the keyword.) All tournament-legal cards with cumulative upkeep printed on them started in Ice Age and ended in Weatherlight, plus a revival in Coldsnap. 
From the Comprehensive Rules:
- 702.23. Cumulative Upkeep
- 702.23a Cumulative upkeep is a triggered ability that imposes an increasing cost on a permanent. “Cumulative upkeep [cost]” means “At the beginning of your upkeep, if this permanent is on the battlefield, put an age counter on this permanent. Then you may pay [cost] for each age counter on it. If you don’t, sacrifice it.” If [cost] has choices associated with it, each choice is made separately for each age counter, then either the entire set of costs is paid, or none of them is paid. Partial payments aren’t allowed.
A creature has “Cumulative upkeep or ” and two age counters on it. When its ability next triggers and resolves, the creature’s controller puts an age counter on it and then may pay , , , or to keep the creature on the battlefield.
A creature has “Cumulative upkeep—Sacrifice a creature” and one age counter on it. When its ability next triggers and resolves, its controller can’t choose the same creature to sacrifice twice. Either two different creatures must be sacrificed, or the creature with cumulative upkeep must be sacrificed.
- 702.23b If a permanent has multiple instances of cumulative upkeep, each triggers separately. However, the age counters are not linked to any particular ability; each cumulative upkeep ability will count the total number of age counters on the permanent at the time that ability resolves.
A creature has two instances of “Cumulative upkeep—Pay 1 life.” The creature currently has no counters but both cumulative upkeep abilities trigger. When the first ability resolves, the controller adds a counter and then chooses to pay 1 life. When the second ability resolves, the controller adds another counter and then chooses to pay an additional 2 life.
- Paying cumulative upkeep is always optional. If it's not paid, the permanent with cumulative upkeep is sacrificed. Partial payments of the total cumulative upkeep cost can't be made. For example, if a permanent with "cumulative upkeep " has three age counters on it when its cumulative upkeep ability triggers, it gets another age counter and then its controller chooses to either pay or sacrifice the permanent.
- Examples of choices associated with cumulative upkeep costs include choosing an opponent, choosing a creature, choosing a color of mana, choosing a graveyard, and choosing "heads" or "tails" when flipping a coin. Each choice within a single payment is made independently. The entire set of choices must be legal and payable.
- Some permanents have abilities that trigger when their cumulative upkeep is paid. These abilities trigger when an entire upkeep payment is made. They don't trigger once per individual cost payment.
- Several cards with cumulative upkeep do something based on the number of age counters on them when they're put into a graveyard from play. For example, Arctic Nishoba says "When Arctic Nishoba is put into a graveyard from play, you gain 2 life for each age counter on it." That ability triggers no matter how the Nishoba is put into a graveyard from play. If you don't pay its cumulative upkeep and have to sacrifice it, it gets an age counter first, which impacts the amount of life you gain.
- Note that the decision to track cumulative upkeep using counters was made when the "Sixth Edition rules" (and the original version of the Oracle card database) were released in 1999. The Coldsnap set is the first since that time to feature cumulative upkeep cards, so this may appear to some players to be a rules change.
- Uktabi Efreet -
Creature — Efreet
Cumulative upkeep (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.)
- Inner Sanctum -
Cumulative upkeep — Pay 2 life. (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.)
Prevent all damage that would be dealt to creatures you control.
 Color percentages
There is a total of 86 cards that involve cumulative upkeep, which divide by color as such:
- Blue = 29 %
- Green = 21 %
- White = 17 %
- Black = 13 %
- Red = 9 %
- Artifact = 5 %
- Multicolor = 3 %
- Land = 2 %
 Enchantments that impose Cumulative upkeep