Basal melt of ice shelves is not only an important part of Antarctica’s ice-sheet mass budget, but it is also the origin of one of the most peculiar types of sea ice found in the polar oceans: platelet ice. In many regions around coastal Antarctica, tiny ice crystals form and grow in supercooled plumes of Ice Shelf Water, releasing heat into the surrounding ocean. They usually rise towards the surface, eventually becoming trapped under an ice shelf as marine ice. Frequently, masses of those crystals are advected out of the ice-shelf cavity, and accumulate below a solid sea-ice cover to form a semiconsolidated layer. When the overlying sea ice grows into this so-called sub-ice platelet layer, the loose crystals are consolidated, adding additional thickness to the sea ice. These phenomena are generally referred to as platelet ice, although confusion about the terminology is widespread in the literature. The presence of platelet ice has a profound impact on sea-ice properties and processes in several regions of Antarctica, with numerous implications for the local polar marine biosphere. Most notably, sub-ice platelet layers provide a stable, sheltered, nutrient- and food-rich habitat which usually results in a highly productive and uniquely adapted ecosystem. It has also been hypothesised that platelet ice may be an indicator of the state of an ice shelf, although comprehensive time series are limited to the Ross Sea. This paper clears up the terminology by providing exact definitions of the relevant terms.We review platelet-ice formation, observational methods as well as geographical and seasonal occurrence. The physical properties and ecological implications are merged in a way understandable for physicists and biologists alike, to lay the foundation for the interdisciplinary research that is necessary to tackle the current knowledge gaps.