A character is Killed Off for Real and assumed by all other characters to be dead, but the death occurs in such a way that no body is recovered. No matter how all laws of physics and biology indicate No One Could Survive That, this old rule trumps all: "Never count someone dead unless you have the body in front of you." (And in some cases, not even then.) A common aversion of this trope can be seen in shows where Everything Fades; if there IS a body to be found, try not to get too used to his absence. See also Left for Dead, when the body is seen but left behind without confirming it's dead.