Refers to sake that has been gently heated, typically served in winter or with certain dishes.
寒い夜には燗酒が一番だ。
On a cold night, warmed sake is the best.
Compound of 燗 (kan, 'warming') and 酒 (sake, 'sake'). The reading かんざけ is a straightforward combination of the on'yomi of 燗 and the kun'yomi of 酒.