Any non-linear shaped opening enclosed by ice.  Climatological term referring to the extreme minimum or extreme maximum extent of the ice edge in any given month or period based on observations over a number of years. This term should be preceded by minimum or maximum.
Environment and Climate Change - Canada https://www.ec.gc.ca/glaces-ice/default.asp?lang=En&n=2CE448E2%E2%80%931&offset=4&toc=hide