-our

-oura suffix of nouns denoting state or condition, a quality or property, etc., as in ardour, colour, honour, labour.

Also, -or. [Middle English, from Anglo-French (= French -eur), from Latin -or -or¹]

Usage: Many words ending in -our, such as colour, honour and vigour, can also be spelt with -or. This variation arose in England in the 17th century, when some theorists believed words ought to be spelt according to their origins. The words from French were supposed, in this case, to be spelt with -our and those directly from Latin with -or. As people were not always sure from which language a word came, there was some confusion, and more and more people felt it best to use -or for all of them. The trend ran its full course in the US where -or is always used. However, it was halted in England by Samuel Johnson`s dictionary of 1755. He allowed some of these words, such as error, horror and terror, to go to -or, but the rest were fixed with -our.

In Australia, as in Britain, the most common spelling of these words is with -our, although -or is often used and certainly occurs consistently in a large number of magazines and newspapers.