What term describes a glacier with a thick center that thins out towards the edges?

The term that describes a glacier with a thick center that thins out towards the edges is known as an ice sheet. Ice sheets are vast expanses of glacial ice that cover land and are usually characterized by their thickness in the center, which arises from the accumulation of snow and ice over time due to compressive flow. As the ice moves outward, it gradually thins toward the margins where melting or calving can occur.

In contrast, an ice shelf refers to a thick, floating platform of ice that extends from land into the sea and is not defined by a thickened center tapering toward the edges. A crevasse is a deep crack in a glacier's surface and does not pertain to the overall structure of the glacier. Additionally, the zone of ablation describes the lower section of a glacier where ice loss occurs primarily through melting and sublimation, rather than referring to the glacier's thickness profile. Thus, the features of an ice sheet accurately embody the description provided in the question.