I'd point out that a knife certainly isn't lethal to an elephant. A .22 isn't lethal to a bear (or at least it's really, really unlikely to be). And, I'd point out that [MENTION=23751]Maxperson[/MENTION]'s own dictionary definition "sufficient to cause death." requires death to be caused for something to be described as lethal.
Well, something might be sufficient to cause death and not cause death because (eg) no one was exposed to it.See, the term "lethal" is a bit fuzzy.
I mean, a single stab wound from a knife is rarely lethal. It can be and certainly it potentially can kill you more often than a single punch or kick, but, by comparison, is far less lethal than, say, being hit by a bus.
My point being lethality is a range from, really not likely to kill you to Stage 5 pancreatic cancer. Claiming that spell damage is "always lethal" doesn't really match up with the mechanics of 5e or, really, any version of D&D. 3e had the separation of lethal vs non-lethal damage, but, that's something of an outlier and, frankly, didn't work very well either.
4e and 5e have no such distinction. Damage is damage. End of story.
And something might even by reasonably described as sufficient to cause death and yet not cause death to someone exposed to it because some atypical countervailing factor came into play (eg the poison is lethal but the victim had taken a prophylactic/antidote; the atomic bomb is lethal, but the target was Superman; etc).
But for the reasons you give this is all completely tangential to the 4e (and 5e) rules. If by lethal we mean literally causing death then a fireball that doesn't kill its target was not lethal, end of story, with no need to distinguish lethal and non-lethal damage and no time travel involved. And if by lethal we mean sufficient to cause death under typical circumstances then it should be no surprise that some people survive - there circumstances obviously weren't typical! We still don't need a distinction between lethal/non-lethal damage and still no time travel is involved.
It baffles me that there's any debate here. The rule is completely straightforward: zero hp for a GM-side creature/character triggers a choice of result by the player of the successfully attacking character. That's one of the clearest rules ever found in a D&D rulebook!