A decrease in the length of a glacier compared to a previous point in time. As ice in a glacier is always moving forward, its terminus retreats when more ice is lost at the terminus to melting and/or
calving than reaches the terminus. During retreat, ice in a glacier does not move back up the valley.
Decrease of the length of a flowline, measured from a fixed point. In practice, when the retreat is of a land-terminating glacier terminus, the fixed point is usually downglacier from the terminus, th
at is, on the glacier forefield. The quantity reported is most often the amount of retreat rather than the length itself. Advance is the opposite of retreat, that is, advance of the terminus.
A glacier whose terminus is increasingly retreating upvalley compared to its previous position due to a higher level of ablation compared to accumulation.
Retroarc (Retro) foreland basins occur on the plate that overrides during plate convergence or collision (i.e. situated behind the magmatic arc that is linked with the subduction of oceanic lithospher
e). [Wikipedia]