Gillick v West Norfolk and Wisbech Area Health Authority  AC 112 was a landmark House of Lords judicial review which established the Gillick competence test, used to determine if a child (aged under 16) can consent to medical treatment without parental permission.
In his judgment, Lord Scarman said:
[…] I would hold that as a matter of law the parental right to determine whether or not their minor child below the age of 16 will have medical treatment terminates if and when the child achieves a sufficient understanding and intelligence to enable him or her to understand fully what is proposed. It will be a question of fact whether a child seeking advice has sufficient understanding of what is involved to give a consent valid in law. AC 112
This is generally what is considered to be meant by ‘Gillick competence’.