Echo is a keyword ability that causes you to pay for a card in instalments. When you first play the card, you pay its mana cost, which is usually less than what a card of that power level would normally cost;; then, during the upkeep of your next turn, you must then sacrifice the permanent unless you pay another cost, the echo cost.
Echo is a triggered ability. "Echo [cost]" means "At the beginning of your upkeep, if this permanent came under your control since the beginning of your last upkeep, sacrifice it unless you pay [cost]."
When echo was first introduced, it was just written as a single word, "echo", with no cost listed. For these cards, the echo cost was identical to their mana cost, and they have received errata to say so explicitly.
- Usually you will only need to pay the echo cost the turn after you cast the card. However, since the ability checks only when it came under your control, you may need to pay it in other circumstances too. For example, if you use Mind Control to take a creature with echo from your opponent, you will need to pay the echo cost during your next upkeep step or else sacrifice the creature.
- . . . with cost reduction effects: Most effects which increase or reduce costs won't have any impact on echo costs. The one exception is Thick-Skinned Goblin, which lets you pay :0m: instead of the card's normal echo cost.