Luck

Luck is part of the main stats, also called SPECIAL-Co values.

In these rules it's usually abbreviated with a single "L".

Luck is the strangest of all stats. Apart from your character's skill with games and gambles, it basically only influences one other derived stat: his (or her) chance to score a critical hit in combat. But it gets tested a lot for other things as well, and therefore influences everything a little bit.
Luck is therefore the most important stat.