-or1 Also -our.
Someone or something that performs a given action.
[From Latin, sometimes via Anglo-Norman French -eour or Old French -eor.]
A wide variety of such nouns exist in English. A few examples in which it refers to a person are actor, author, commentator, creator, doctor, editor, inventor, navigator, oppressor, sculptor, translator, and visitor. Some of those in which it refers to a device or machine are accumulator, capacitor, carburettor, motor, percolator, refrigerator, reflector, rotor, and tractor.
The ending is for most purposes identical in sense and usage with -er (see -er1). There is a tendency for words in -or to be preferred for an agent that is an intangible entity (descriptor; sensor, vector) and in legal terms (lessor, mortgagor, vendor), for which matching terms in -ee usually exist for the other party to the transaction. However, differences are slight and there is no rule to determine which ending is correct in any given case. Both the forms adviser and advisor are widely used, through the former is much more common in British English.
A very few such words are spelled -our in British English, particularly saviour, though this spelling is also sometimes used in American English when it refers to Jesus Christ.