The building blocks of English

Affixes

-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).