It's a word that's been used for centuries but according to one of the world's leading writing style guide describing a woman as a "mistress" as it implies she is "solely responsible for the affair".
According to Merriam-Webster dictionary a mistress is a "woman other than his wife with whom a married man has a continuing sexual relationship".
"Don't use the term mistress for a woman who is in a long-term sexual relationship with, and is financially supported by, a man who is married to someone else," the Associated Press Stylebook tweeted on Tuesday.
Alternatively, the style guide said, you should use words like "companion, friend or lover" as the first reference and add extra details afterwards.