Echo

[ibtrickey]

What does echo mean on a card? {Deepcavern Imp}

[Gorzo]

Echo is a keyword that means on your next upkeep, the turn after you cast the permanent that has echo, you must pay the specified echo cost or sacrifice the permanent. It is a 1-time trigger, and you only have to do it the one time, on that next upkeep.

RESOLVED

[Paraluke]

When echo first appeared, it was usually the casting cost - hence if you see cards with just echo as keyword, I believe it meant to pay the casting cost on next upkeep or you would have to sacrifice the permanent.

[Pleeb]

If I were to steal a creature with echo, would they have to pay the echo cost again after it goes back under their control?

[Keyeto]

If I were to steal a creature with echo, would they have to pay the echo cost again after it goes back under their control?

Yes. Echo cares about whether it came under your control since your last upkeep, not whether it entered the battlefield since then:

702.28a 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]."