Natural 1
When the twenty-sided die itself shows 1, before any modifiers. Some games treat it as an automatic failure.
A natural 1 is the worst a twenty-sided die can show. Some rulesets make it an automatic miss or failure no matter how big your modifiers are; others treat it like any other low roll.
In Fantasy RPG
Fantasy RPG: on an attack roll a natural 1 always misses, whatever the bonus. On checks and saving throws it is just a low roll.