A legal doctrine is a rule, interpretation, theory, practice or policy that judges use while determining a case and that lawyers apply while advocating for their clients.
Law is often articulated in written rules or statutes, or enumerated by judges in written opinions following decisions made in specific cases. Language is often open to interpretation, and a legal doctrine is developed to aid in legal interpretation.
Law is best understood as the application of rules, and a legal doctrine is a guideline for understanding how to apply those rules. It is impossible for a system of law to anticipate every scenario in which the law will be applied.
A legal doctrine forms over time to help judges understand how to decide a case, to help lawyers decide how to argue a case, and to help the public predict how the justice system will view a proposed course of action.