-ator

American  

1. a combination of -ate and -or that forms nouns corresponding to verbs ending in -ate1 ,denoting a human agent (agitator; mediator; adjudicator ) or nonhuman entity, especially a machine (incubator; regulator; vibrator ) performing the function named by the verb.

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-ator British  

suffix

1. a person or thing that performs a certain action

   agitator

   escalator

   radiator

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Etymology

Origin of -ator

< Latin -ātor, originally not a suffix, but the termination of nouns formed with -tor -tor from verbs whose stems ended in -ā-; in English, Latin loanwords ending in -ātor have been reanalyzed as derivatives of the past participles in -tus ( -ate ¹ ) and a suffix -or ( -or ² ), and many new English nouns derived from English verbs based on Latin past participles (e.g., vibrator from vibrate )