Nigori-zake (にごり酒) is white, cloudy sake. Sake is made by straining the “moromi (もろみ, fermenting mash)” after fermentation has finished, separating it into liquid and sake lees (sake kasu, 酒粕). Nigori-zake is white and cloudy because the moromi has only been lightly strained, through a coarse cloth or something similar.
How it differs from doburoku
Under the law, seishu (清酒, sake) is made by fermenting rice, rice koji and water and then “straining” the result. Nigori-zake is strained, even though it is through a coarse cloth, so it is seishu. Moromi that has not been strained at all is called “doburoku (どぶろく)”, and it is not included in seishu.
How it differs from ori-zake
Ori-zake (おり酒) is also white, cloudy sake, but it is made differently. Even when the moromi is strained through a fine cloth, if the pressed sake is left to stand, a white sediment called “ori (おり)” collects at the bottom. Ori-zake is white, cloudy sake that contains this ori, sold as a product.
Nigori-zake that has not been pasteurized
Some nigori-zake has been through pasteurization (hiire, 火入れ: a process of heating the sake to sterilize it and keep its quality stable), and some has not. In nigori-zake that has not been pasteurized, the yeast is still alive. If it is kept at room temperature, fermentation continues and carbon dioxide gas may be produced. The labels of sake that has not been pasteurized give notes on storage and on how to drink it, so please follow them. Pasteurization is explained on the Namazake page.