What is the definition of plate boundaries?

It is when 2 plates are moving away from each other and new crust is forming from magma that rises to earth's surface between 2 plates.
hope it helps

Magma forms by partial melting of upper mantle and crust. partial melt means that only a fraction of the available material forms a melt, and that the remainder stays solid. the partial melt rises because of its lower density and ascends through he crust. this is so conclusive mean the answer would be magma igneous rocks or balistic magma.