Magic: the Gathering rule that was introduced in the Ice Age expansion.
Cards with an effect that says "Cumulative Upkeep: something" mean that the upkeep cost is paid every turn, and in addition you also need to pay the upkeep cost you paid the previous turn.
For example, "Cumulative Upkeep: 2" would mean that you need to pay 2 mana in the next upkeep phase, then 4 in the next turn's upkeep phase, then 6, then 8... And, of course, if this cost is not paid, the card is history.
This was, of course, a great way to balance some really powerful cards - you have the Might, but only for a limited time. The rule was objected by some people, but it's still fairly easy to understand and works pretty well. The number of cards with this ability appear to be in decline, though.
(For what it's worth, the only card with cumulative upkeep I have in my deck is Arctic Wolves... A great 4/5 way to frighten the little sister.)