noun
A rubber or plastic bag filled with ice, used for cooling injuries or reducing fever.
熱があるので、氷嚢で頭を冷やした。
I had a fever, so I cooled my head with an ice bag.
捻挫した足首に氷のうを当ててください。
Please apply an ice pack to the sprained ankle.
保冷剤 is a reusable cooling gel pack, while 氷嚢 specifically refers to a bag filled with ice.
Compound of 氷 (ひょう, 'ice') and 嚢 (のう, 'bag' or 'sac'). The reading のう for 嚢 is the on'yomi.