The unwritten rule of tabletop Role Playing Games (but can be applied elsewhere). It says:
"Don't be a dick."
It essentially means players should treat each friendly and with respect.
If someone intentionally does something inappropriate, or acts maliciously towards another player or players, he's breaking Rule 0.
Particular behavior breaking the rule may differ according to a game you are playing. For example, some RPGs are designed as competitive games, while others are cooperative. Behaviour common in the first type could be sometimes considered bad (breaking the Rule 0) in the latter.
In most games, killing another player's character without proper reason* qualifies as breaking Rule 0.
* other than "I wanted your loot / I wanted to earn XP by killing you / I did it for the lulz" etc...
Ther term comes from the British prisonsystem practice of segregating Sex Offenders under Rule 45, to stop them being attacked by other inmates. (Formerly Rule 43)
"If I could get onto that Wing with a razor blade, I'd have a ball. Or possibly a handful of balls, I hate Nonces "
"Yes, but Governor, you are obliged to protect those Rule 45 boys"
Most commonly used in quiz or exam multiple choice situations.
After trying to figure out a numerical problem and getting the wrong answer BUT you are in the ballpark area, divide or multiple by 2 to get your answer.
If you have no idea how to get the right answer the right way, it is in your interest to use the rule of 2 .
This rule also works with 10, but that is more commonly used for conversion errors (metric).
I couldn't get 1250 for the life of me! I kept getting 1250.