Marriage - permanent
The relation between man and woman becomes lawful by contracting marriage. There are two kinds of marriages:
In a permanent marriage, the period of matrimony is not fixed, and it is forever. The woman with whom such a marriage is concluded is called da'ima (i.e. a permanent wife).
In a fixed time marriage (Mut'ah), the period of matrimony is fixed, for example, matrimonial relation is contracted with a woman for an hour, or a day, or a month, or a year, or more. However, the period fixed for the marriage should not exceed the span of normal lives of the spouses, because in that case, the marriage will be treated as a permanent one. This sort of fixed time marriage is called Mut'ah or Sigha.