The Big Bad TV trope
The cause of all bad happenings in a story. The Big Bad may either be personally responsible for the events, or the biggest force in opposition of the hero's goals. A Big Bad could be a character with Evil Plans or it could be an omnipresent situation, such as a comet heading towards the Earth. In a serial story, the Big Bad often exerts an effect across a number of episodes, and even an entire season. In a standalone cinematic story, their presence drives the plot.
A Big Bad is not a catch-all term for the biggest, ugliest or even primary villain of any given story. The Badass leader of the outlaw gang that causes the most personal trouble is not the Big Bad. The railroad tycoon who turns out to be using the gang as muscle is the Big Bad. The Man Behind the Man is very common for this trope, leaving the reveal of the big bad as The Chessmaster behind it all and proving themselves far more clever and resourceful than the Villain of the Week. Sometimes the Big Bad is the grand enemy of the entire story as an Overarching Villain. At other times, the Big Bad is an Arc Villain who causes trouble for a period of time only to be replaced by another Big Bad with ambitious plans. The Big Bad may be confronted frequently, but is too powerful to finish off until the last episode of the story arc. The Big Bad may work through Evil Minions and will almost certainly have The Dragon protecting them, to keep interest up and provide something for the good guys to defeat. When you look at a season-long story or a major Story Arc and you can identify that one villain as being the one in control of everything, that is the Big Bad.
In its most general terms, a Big Bad will be at the center of the Myth Arc rather than just any Story Arc. At the same time, the Big Bad is not exclusively the most dangerous enemy of the arc. In many cases, you will find that while the Big Bad may be in control, the Dragon-in-Chief would still be the greater threat.