As defined by Article II of the ABM Treaty:
A system to counter strategic ballistic missiles in flight, and consisting of:
- ABM interceptor missiles
- ABM launchers
- ABM radars, which are radars constructed and deployed for an ABM role, or of a type tested in an ABM mode
By Agreed Statement(D) at the signing of the ABM Treaty this definition was recognized to be a bit too restrictive, and so:
In order to insure fulfillment of the obligation to not deploy ABM systems and their components except as provided in Article III of the Treaty, the Parties agree that in the event ABM systems based on other physical principles and including components capable of substituting for ABM interceptor missiles, ABM launchers, or ABM radars are created in the future, specific limitations on such systems and their components would be subject to discussion in accordance with Article XIII and agreement in accordance with Article XIV of the Treaty.
In short, an ABM system, is a collection of linked components designed to intercept and destroy strategic ballistic missiles.