Parens patriae (Latin): A British common law creation whereby the courts have the right to make unfettered decisions concerning people who are not able to take care of themselves.
For example, court can make custody decisions regarding a child or an insane person, even without a statute to allow them to do so, based on their residual, common law-based parens patriae jurisdiction.
Judicially:
"From the earliest time, the sovereign, as parens patriae, was vested with the care of the mentally incompetent. This right and duty ... is founded on the obvious necessity that the law should place somewhere the care of persons who are not able to take care of themselves.
"In early England, the parens patriae parens patriae jurisdiction was confined to mental incompetents, but its rationale is obviously applicable to children and, following the transfer of that jurisdiction to the Lord Chancellor in the seventeenth century, he extended it to children under wardship, and it is in this context that the bulk of the modern cases on the subject arise. The jurisdiction was later vested in the provincial superior courts of this country."
And:
"The parens patriae jurisdiction is ... founded on necessity, namely the need to act for the protection of those who cannot care for themselves. The courts have frequently stated that it is to be exercised in the "best interest" of the protected person, or again, for his or her "benefit" or "welfare". While the Superior Court retains a residual jurisdiction to use the parens patriae power, it will not do so lightly. This jurisdiction is to be exercised to protect children and other vulnerable individuals, not their parents. The courts have determined that parens patriae is available in two situations: to fill a legislative gap or on judicial review."