From Longman Dictionary of Contemporary Englishparamourpar‧a‧mour /ˈpærəmʊə $ -mʊr/ noun [countable] literarySYRELATIONSHIP someone who you have a romantic or sexual relationship with, but who you are not married to SYN lover
Examples from the Corpus
paramour• They bade both the merchant and his paramour adieu.Origin paramour (1300-1400) Anglo-French par amour “by way of love” 