A legal power gives a legal subject the ability to perform juridical acts that have the legal effect of changing the legal status of itself and/or other legal subjects.
Legal powers highlight the fact that law is not just about regulating what legal subjects can do, as is often assumed, but also about empowering them to create new states of enforceable legal effect.
Public legal powers enable authorities to, for example, legislate (parliament) or adjudicate (courts and tribunals). Parliament has the legal power, for example, to impose a universal speed limit on public roads within its jurisdiction.
Private legal powers enable legal subjects to, for example, conclude contracts of sale, employment, or marriage. They also enable the creation of new legal subjects such as companies, charities, or indeed children (that is, the legal subjectivity of the child that is born).
Legal powers often accrue as the result of holding subjective rights. For example, one has the legal power to sell one’s car (power of disposition) because one owns it (the subjective right of ownership entails the power of disposition).
Legal powers are generally dependent on the legal subject having the capacity necessary to exercise them. For example, a minor or someone with serious cognitive impairments will be unable to conclude a marriage contract, due to lack of majority age or the ability to give fully-informed consent, respectively (in some jurisdictions both conditions will apply to the minor).