Natural 1

Also known as a "Critical miss", or a "critical failure". The worst roll that can happen on a 20 sided die: a 1 on the die. This means that, if rolled for an attack, the attack automatically misses no matter what modifiers the roll may have. This has no real special meaning for anything other than an attack, but some games allow this to mean that what ever the character is trying to do automatically fails, no matter how good the character may be at what they may be trying to do.

This is counter to a Natural 20