I can see that most of you don't understand what endgame stands for.
It doesn't mean that the game ends. It does mean that there is still something to do after completing main story, levels, defeating final boss, having all applicable content at max level etc.
What I (and most of players who understand this term) mean by something is in most cases progression (getting new, special equipment), unlocking special levels, dungeons, enemies, events, grinding for items which can give you new equipment and so on.
Examples:
Diablo 3 - has a pretty bad end game, most Diablo players don't like it but it still there and there is lots to do. Leveling paragons (unlocks after getting final 70 level), grinding/farming sets, killing bosses on higher difficulties, crafting gemstones (which cost lots of gold), doing bounties and rifts which every level get harder.
Dying Light - farming new guns, schematics, completing achievements, side quests, Bozak hordes, Invasions, leveling up, collecting flags, rocks, trophies.
Terraria - farming bosses for special equipment, trophies, surviving Frost Moon & Pumpkin Moon, eliminating Hallow, Crimson/Corruption, building (which is many much, much better than what we have got in 7 days to die).
Even games like Smash Bros, Mario Galaxy, Monster Hunter, Fire Emblem, Fantasy Life don't end after beating main story. In some of them you actually spend more times at the end game. Of course don't forget about RPGs...
...or even MMORPGs.