Refers to milk with the fat removed. Common in dietary and nutritional contexts.
I use non-fat milk in my coffee.
低脂肪乳 is low-fat milk, which still contains some fat, while 無脂肪乳 has virtually no fat.
牛乳 is regular whole milk, which contains natural milk fat.
Compound of 無 (mu, 'without') + 脂肪 (shibō, 'fat') + 乳 (nyū, 'milk').