According to Hindu Law, the definition of marriage is not a contract but a sacrament. A sacrament is a religious ceremony that is believed to have a divine presence and significance. Hindu marriage is considered to be a sacred union between two individuals, with the presence of gods and goddesses as witnesses. It is not categorized as a mere contract but holds a spiritual and religious significance.

The other options provided in the question are incorrect. The statement that marriage is a mutual understanding is not a correct definition according to Hindu Law. While mutual understanding is desired in a marriage, it does not define the nature of marriage itself. Similarly, the statement that marriage is indissoluble is not entirely accurate according to Hindu Law. While Hindu marriages are considered to be long-lasting and meant to be lifelong, there are provisions for divorce and dissolution of marriage under certain circumstances.

Therefore, the correct answer is option 2 - 2 only, as per Hindu Law, marriage is defined as a sacrament.