second-year ice
Technical term in glaciology and oceanography for sea ice that has survived one summer's melt and is in its second winter of growth.
In the Arctic Ocean, second-year ice is widely distributed.
多年氷 is multi-year ice that has survived at least two summers, making it even thicker and harder than 二年氷.
Compound of 二年 (two years) and 氷 (ice), literally 'two-year ice'. The reading ごおり is a rendaku form of こおり (氷).