OGL content from Swords and Wizardry ( https://www.d20swsrd.com/for-players/characters/armor-class/ )
From time to time, a spell or some other kind of hazard requires you to make a “saving throw.” A successful saving throw means that the character (or monster) avoids the threat or lessens its effect. Each character class has a “Saving Throw” target number, which gets lower and lower as the character gains levels. To make a saving throw, roll a d20. If the result is equal to or higher than the character’s saving throw target number, the saving throw succeeds. Monsters can also make saving throws (a monster’s saving throw target number is listed in the monster’s description).