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common-law marriage
n. an agreement between a man and woman to live together as husband and wife without any legal formalities, followed and/or preceded by cohabitation on a regular basis (usually for seven years). Common-law marriage is legal in Alabama, Colorado, Georgia, Idaho, Iowa, Kansas, Montana, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Carolina, Texas and Utah, thereby recognizing a marriage for purposes of giving the other party the rights of a spouse, including inheritance or employee benefits. Such informal partnerships are recognized by some local governments for purposes of the rights of a spouse under employment contracts and pension rights even where the state does not recognize this as a marriage.

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cohabitation
n. living together in the same residence, generally either as husband and wife or for an extended period of time as if the parties were married. Cohabitation implies that the parties are having sexual intercourse while living together, but the definition would not apply to a casual sexual encounter. Legal disputes have arisen as to whether cohabitation would refer to same sex partners, which is important to those involved since "cohabitation" is the basis of certain rights and privileges under various laws, regulations and contracts. The findings of the courts vary on this question, but the trend is to include long-standing homosexual relationships as cohabitation.

Prenuptial Agreement: http://en.wikipedia.org/wiki/Prenuptial_agreement

A prenuptial agreement or antenuptial agreement, commonly abbreviated to prenup or prenupt, is a contract entered into by two people prior to marriage or civil union. The content of a prenuptial agreement can vary widely, but commonly includes provisions for the division of property should the couple divorce and any rights to spousal support during or after the dissolution of marriage.

Prenuptial agreements are a matter of civil law, so Catholic canon law does not rule them out in principle (for example, to determine how property would be divided among the children of a prior marriage upon the death of one spouse).

In practice, prenuptials may run afoul of Church law in a number of ways. For example, they cannot subject a marriage to a condition concerning the future (such as an agreement about the dividing of assets in case of divorce). The Code of Canon Law provides: "A marriage subject to a condition about the future cannot be contracted validly." (CIC 1102)

The Canon Law: Letter and Spirit, a commentary on canon law, explains that condition may be defined as "a stipulation by which an agreement is made contingent upon the verification or fulfillment of some circumstance or event that is not yet certain." It goes on to state that "any condition concerning the future attached to matrimonial consent renders marriage invalid." For example, a marriage would be invalid if the parties stipulated that they must have children or they have the right to divorce and remarry someone else.

Everything you need to know about prenuptial agreements : http://www.bankrate.com/brm/prenup.asp
By Bankrate.com

Nothing can kill romance faster than the word prenup. But with about one in three of all first marriages ending in divorce, and 50 percent of second or third ones hitting the skids, a prenup is smart financial planning, legal and financial experts say.

"Think of it as a business arrangement or as an insurance policy to help remove some of the emotion that's naturally involved," says Nancy Dunnan, a New York City financial adviser and author. "Marriage is not just an emotional and physical union -- it's also a financial union. A prenup and the discussions that go with it can help ensure the financial well-being of the marriage."