乳 　　rǔ 　分泌奶的器官：～房（亦称“乳”、“奶子”）。～峰。～罩。 　　乳房中分泌出来的白色甜汁：～汁（亦称“奶”）。哺～。～母（奶妈）。～臭（xiù）未干（对年幼人表示轻蔑）。 　　像乳汁的东西：豆～。～胶。～腐（亦称“腐乳”）。 　　像乳头的东西：钟～（钟上可敲打的突出物）。钟～石。 　　生，生殖：孳～。 　　初生的、幼小的：～燕。～牙。～名（小名）。 　　breast　breed　milk 　 奶　nǎi 。字从女，从乃，乃亦声。“女”指成年女性。“乃”为“孕”省。“孕”意为“重复生子”。“女”和“乃”联合起来表示“连续多年生子的妇女”。本义：生育了一群孩子的母亲。转义：1.哺育期妇女。2.乳汁。3.用乳汁哺育孩子。喂乳。4.乳头。乳房。本义的引申义：祖母。

乳和奶的区别，就像母亲和妈妈的区别，乳更书面和正式，奶更口语化。 有的时候可以相互代替，但是有些搭配已经成为习惯并且固定下来，你只能见一个记一个了。 这里有个语感，比较正式或者专业的名称，你就用乳，生活中口语中常提到的名称，你就用奶。固定搭配除外。 这个语感，可以扩展到其他你容易混淆的书面用词和口语用词。 ru3 is more formal than nai3, just like mother and mom. there is a feeling for selecting how to use ru3 and nai3. when the situation or the context is more formal, you use ru3; otherwise, use nai3. but if that collocation of word is fixed, you must recite it.

When they both reffer to as "milk", “乳” is more in writing chinese, and "奶" is more in speaking chinese.

Generally speaking, 乳ru3 is a word in ancient Chinese and a word-forming morpheme in modern Chinese , and 奶nai3 is a modern word and morpheme. In modern Chinese, ru3 is used only as a free morpheme to form compound words, such as 乳房（mamma, breast, udder ）、乳头( mammilla, nipple, teat )、乳腺（mammary gland), 乳罩（aslo 胸罩, brassiere ) , 乳汁（milk) ; 乳名（ a Child's pet name); 乳制品（dairy produce), 乳品厂（dairy/milk plant), 乳脂（milkfat); 乳糖（lactose). Ru3 is chiefly used in science/technology, industry and literature. 奶nai3 is commonly and frequently used in daily Chinese. You say 奶子（vulgar, privately used) but not 乳房；奶头，but not 乳头； 小名，but not 乳名；奶水( milk, concerning mothers' supply to babies )，but not 乳汁（no 乳水); 吃奶, not 吃乳；牛奶, not 牛乳; 奶粉, not 乳粉; 奶瓶，not 乳瓶, etc. But nowadays except scientific terminology, nai3 is used more than before. For example, 乳罩 can also be called 奶罩，乳酪 also 奶酪, 乳制品 also 奶制品, etc. Etymologically, 乳ru3 and 奶nai3 are cognate words. ------------------------------------------------- ▲ 现代汉语词典（第五版） http://product.dangdang.com/product.aspx?product_id=20264168 ▲ 现代汉语词典（第四版）（The Contemporary Chinese Dictionary: Chinese-English edition) http://product.dangdang.com/product.aspx?product_id=706098 ▲ 新华字典（第十一版） http://product.dangdang.com/product.aspx?product_id=21114193

Besides milk, both 奶 and 乳 also mean breast and breast-feed. 奶 is used frequently than 乳, while 乳 is more ceremonial and elegant.