The property that each spouse brings into the marriage, that is, the property that s/he owned before the marriage, is considered to be "separate" or "non-marital" property. For the property to remain separate, the spouse must keep it apart from marital or community property; that is, s/he would keep it entirely in his/her name. Once the separate property has been commingled (mixed) with marital or community property, it becomes part of the marital property.
For example, consider a bank account with $10,000 in it owned by woman before her marriage. This woman then marries and both she and her husband regularly deposit their respective paychecks into the account and periodically withdraw money to pay for their living expenses. At the time of separation twenty years later, the bank account has $5,000 in it. Since marital property has gone
into it (deposits of the paychecks and marital or community debts have been paid from it is impossible to trace the original separate property money from that of marital or community property. The result is that this bank account has changed from separate property to marital property.
Some states, such as California, have a separate property rule that says that all property brought into the marriage, (including gifts and inheritance), that is kept separate and apart from community property remains the separate property of the spouse that owns it.