-or2 Also -our.
Forming nouns denoting an abstract state or condition.
[From Latin, sometimes via Old French -or, -ur.]
Examples are error, horror, pallor, stupor, terror, and tremor. Many of these were once spelled -our and this spelling persists in some examples in British English (behaviour, colour, fervour, honour, valour, vigour), which—in part as a result of spelling reforms advocated by Noah Webster—are now spelled -or in the US: behavior, color, fervor, etc.
Some words in British English that are spelled -our lose the u when forming derivatives (clamorous, coloration, glamorize), but others keep it (behaviourist, honourable).