The Tiger Woods "mistress" count is up to 11, the Boston Herald reported on Wednesday. If the golfer wanted to have "mistresses" at every port, he shouldn't have bothered getting married, a contributor at Examiner.com opined recently. And as "alleged mistresses … come out of the woodwork," the Seattle Post-Intelligencer noted Tuesday, once-devoted sponsors are growing wary. It's shocking, really: The more the press covers the Tiger Woods scandal, the more abuse they heap on the word mistress. We don't know much for certain about Tiger Woods' extramarital relations. But the term mistress generally connotes a level of commitment to one's side dish(es) that does not seem to be present here. A woman who has sex with a man once—or even repeatedly— but without any real devotion is not really his mistress.
The word mistress entered English in the 14th century by way of French. Effectively equivalent to master with the ess feminine suffix, it originally meant "a woman having control or authority"—such as a woman who is the head of a household. By the 15th century, the word developed the meaning "a woman who is loved by a man; a female sweetheart," but the specific sense "a woman other than his wife with whom a man has a long-lasting sexual relationship," to quote the Oxford English Dictionary's definition, doesn't appear until the early 17th century. (John Donne made this meaning particularly clear in a sermon mentioning "women, whom the Kings were to take for their Wives, and not for Mistresses, (which is but a later name for Concubines).")
This bare dictionary definition, even with the emphasis on "long-lasting," doesn't fully capture the nuances of mistress's use. A mistress is exclusively devoted to one man. Although that man may have other partners, his relationship with his mistress is relatively serious and stable. He may even pay to support her, or at least help cover some of her living expenses. This signification comes across in characteristic quotations from such authors as Edith Wharton ("Is it your idea, then, that I should live with you as your mistress—since I can't be your wife?"), F. Scott Fitzgerald ("There is always a halt there of at least a minute, and it was because of this that I first met Tom Buchanan's mistress. The fact that he had one was insisted upon wherever he was known. His acquaintances resented the fact that he turned up in popular restaurants with her"), and John Updike ("He phoned the news, not to his wife, whom it would sadden, but to his mistress").
If the type of romantic partnership that mistress evokes seems a little quaint, that points to the very problem with the word in current use: It refers to a social role for women that is increasingly rare, because it is increasingly unnecessary, in modern-day America. A man who is devoted to another woman can divorce his wife without the same social stigma that would once have applied; if his needs are purely recreational, he can engage in casual affairs without doling out serious amounts of cash. Conversely, women no longer need sugar daddies for support. Modern women, for the most part, have access to financial—and sexual—opportunities that make subservience to a married man distinctly less appealing.
This is not necessarily the case in other countries. In France, for example, keeping a mistress is still viewed as a relatively normal thing to do—nowhere better exemplified than the funeral of former President François Mitterrand, at which his wife and two sons stood side by side with his longtime mistress, Anne Pingeot.
Although mistress does not seem to fit the bill for Tiger's ladies, there is no obvious alternative. Girlfriend usually implies an ongoing relationship, as does lover, which is in any case regarded by many media outlets as a bit too explicit. There are also expressions, often slangy, for the relationship itself, including affair (which can, but does not always, imply a continuing relationship), one-night stand, or hookup. These expressions, however, or more circumlocutory descriptions ("a woman with whom Tiger Woods had an affair"), are clunky and therefore not appropriate for headlines.
If we had a female philanderer on the rampage, the situation would be even trickier, because there is no equivalent word for a male in the role of a mistress—it's not a relationship common enough to have generated a term. Paramour can be used of both sexes, but is poetic and somewhat archaic, and isn't limited to the lover of a married person. Cavalier servente is perfect, but only if the relationship is between an unmarried young nobleman and a married noblewoman, and you're in eighteenth-century Italy, thus limiting its use.