「牛乳」 (gyuunyuu) means "milk," specifically cow's milk. It's a white liquid produced by mammals and often consumed by humans. You use 「牛乳」 when talking about drinking milk or using it in cooking. 「毎朝牛乳を飲みます。」 (I drink milk every morning.)

牛からとれる白い飲み物です。カルシウムが豊富で、朝食によく飲みます。