Back-arc basins (or retro-arc basins) are geologic features, submarine basins associated with island arcs and subduction zones. They are found at some convergent plate boundaries, presently concentrated in the Western Pacific ocean. [Wikipedia]
A forearc is a depression in the sea floor located between a subduction zone and an associated volcanic arc. It is typically filled with sediments from the adjacent landmass and the island arc in addition to trapped oceanic crustal material. [Wikipedia]
The remnant arc is what is left on the rear side of the speading zone as athe basin broadens. [Wikipedia]
In a Back-arc basin the arc axis is the line where spreading occurs on the overlying plate of the subduction zone. [Wikipedia]