The phrase "multiyear ice" refers to ice that has been present in the Arctic for more than one year. It is thicker and stronger than ice that forms annually and is more resistant to melting. Full definition
These data were used to quantify changes in the ice cover's composition, revealing a substantial reduction of about 14 percent in the area of multiyear ice in winter during the period from 1978 to 1998. (science.sciencemag.org)
Likewise NOAA's 2014 Arctic Report card also stated the «coverage of multiyear ice in March 2014 increased to 31 % of the ice cover from the previous year's value of 22 %.» (landscapesandcycles.net)
Nevertheless, as pointed out by many others, the fraction of thicker multiyear ice has been declining. (dotearth.blogs.nytimes.com)