Infidelity" (in-fi-del"-i-tee) n"." unfaithfulness or disloyalty to another, esp., sexual unfaithfulness to a husband or wife; an unfaithful or disloyal act. - "Webster's New World Dictionary"

Sex may be the first thing that comes to mind when you hear the word "infidelity," but there are other types of "unfaithful or disloyal acts," other ways spouses cheat on each other.

A major one is money.

In the wake of the revelations regarding former New York governor Eliot Spitzer's trysts with high-dollar call girls, couples - especially women - were giving their own relationships the once-over. In online chats and call-in radio programs, the discussions followed a common theme: "What would I do if that happened to me? Surely I would have had a clue it was coming. And anyway, how could he spend serious money on an affair or a prostitute - or on anything else, for that matter - without my knowing?"

How indeed. Many couples commit monetary deceit in their marriages. Someone lies about finances or doesn't share the details. It can be innocuous, such as fudging on the cost of purchases or hiding a spending spree. Or it could be more significant, such as having a secret credit card or bank account, serious enough to be considered what some relationship experts call "financial infidelity."

The idea of keeping money secrets from a spouse or partner isn't a new one. Sitcoms of the past often played off the theme. "I Love Lucy "drew laughter when Lucy tried to hide her spending from Ricky.

And certainly it doesn't always involve sex, although where there is sexual infidelity, there is most likely financial infidelity as well.

"Everyone in my practice who is committing adultery is committing financial infidelity. It goes with the territory," says Bonnie Eaker Weil, a New York relationship therapist and the author of "Financial Infidelity", to be published April 17.

Those who have sex outside their relationship have to be financially sneaky. Their hanky-panky requires money to entertain a lover or to pay for a prostitute.

Stephany Alexander of Phoenix founded WomanSavers.com, which claims a database to warn women about men who lie, cheat or are abusive. She says it's easier for some people to cheat sexually and financially.

"It's less obvious with people who have a lot of money, like politicians and celebrities," Alexander says. "They are on the road and traveling a lot and have high expense budgets and can cover up a lot that the average person might not be able to."

Most adults - married and single - don't believe financial infidelity is grounds for divorce, but they agree it is a violation of trust, according to a USA TODAY/Gallup Poll of 1,001 adults.

The most heinous breach is a secret bank account: 62 percent of the 557 married respondents say it's a major violation; 11 percent believe it's grounds for divorce. Among the 444 unmarried respondents, 55 percent believe a secret account is a major violation; 13 percent would divorce.

Hiding a purchase from a spouse and keeping a secret credit card are also considered major violations - almost 60 percent say so. But those who are currently married may be slightly more forgiving: Only 6 percent say both are grounds for divorce, compared with about 10 percent of the unmarrieds.

IT'S NOT ALWAYS ABOUT SEX

Even without a sexual liaison, serious financial secrets can be destructive to a relationship.

Financial planner Ginita Wall of San Diego says she has seen countless incidences of financial infidelity.

"It's usually fairly minor - people buying things and saying they had them in the closet for ages or someone goes out with a group at lunch and puts it on the credit card, and they reimburse him in cash."

Wall has seen some extreme cases, including a client in a rocky marriage who for 25 years took cash back at the grocery store every week and ended up with $250,000 in a bank account in her sister's name.

"Her husband thought she was bad at managing household funds. She was always short. He was shocked when he found out she had squirreled that money away."

Divorce lawyer Alton Abramowitz, a vice president of the American Academy of Matrimonial Lawyers, also has tales of financial infidelity, which he says happens "very frequently."

"It could be people going to Las Vegas or Atlantic City and losing money and not telling the spouse," he says. "Or they're not depositing their entire paycheck into the joint account. Or people are having credit card bills sent to the office instead of the house, and the spouse doesn't know they're running up these credit card bills."

Middle-school teacher Chris Matier of Greeley, Colo., says he did just that, running up about $20,000 of debt during an 18-month period on four credit cards that his wife, Tammy, a school administrator, didn't know he had.

"We didn't discuss money at all," he says. "She was out of that loop."

That was eight years ago, following the birth of their first child, when Matier says he was buying "toys," such as video games, computers, CDs and electronics. He had already maxed out the couple's credit card.

"Things started mounting very quickly, and I got caught," he says. "A bill collector from the hospital called looking for money, and I wasn't home. I was out shopping, ironically. There were quite a few bills I hadn't paid: credit cards, hospital bills and tuition payments (for his wife's master's degree).

"MY WIFE CONFRONTED ME ON IT, SO I CAME CLEAN."

The Matiers, both 34, will celebrate their 14th wedding anniversary in May. They plan to buy a house in about 18 months, when both cars will be paid off, they will no longer need day care for their young son, and they'll own the mobile home where they live.

An online survey by Yahoo Finance and the research firm Decipher, released last month, found that 48 percent of the 600 respondents in a serious relationship admitted to some type of financial infidelity.

The survey found 45 percent agreed that spending between $100 and $500 without telling their partner was all right.

Yahoo Finance columnist Laura Rowley says 3 percent said spending up to $5,000 without consulting a significant other is OK.

A LITTLE "MAD MONEY'

What do experts advise couples to do to avoid financial infidelity?

Former accountant Tina Tessina, a psychotherapist in Long Beach, Calif., and author of the new book "Money, Sex, and Kids", says dual-income couples and stay-at-home parents should have some discretionary money to feel "a little autonomous."

How much should a person be able to spend without consulting his or her partner? Amounts vary with the couple's economic status, but Tessina says an average couple should discuss spending more than $200.

Estate adviser Bambi Holzer of Beverly Hills, Calif., agrees. She also says separate bank accounts can work, especially if the partners can't agree on how to spend.

"If you have a male that wants to spend money on a football game and go to a bar with the guys and the woman wants to buy a purse, rather than get permission, they should have an account for minor purchases, not major ones," says Holzer, author of "Financial Bliss."

Bruce Bickel of Pittsburgh, managing director of PNC Financial Services Group's wealth management division, advises against separate accounts and urges couples to overcome that tendency toward autonomy and toward a spirit of dependency on each other.

"I try to prevent that (financial infidelity) rather than correct it," says Bickel, who does suggest a buffer fund from the monthly income that either can access after consulting the other.

That kind of open communication is the way Luke and Hannah Wickham of Orlando handle their finances. Luke, 31, and Hannah, 29, were married five years ago. He had student loans and she didn't. They agreed to talk about any spending over $100.

"If you don't have a major discussion regarding debt and money before you got to the altar or live together and make a major investment together, you're walking down a dangerous path," he says. "You can be rich in love, but money is important in paying the bills."