In chinese, most of words are formed by the combination of characters. a character is a 词素, meaning "element of word". You know H2O is water. So H and O are 词素, H2O is word. So, 眼 and 睛 are 词素. 眼睛 is a word. 词素 normally CAN"T be used alone. the 词素 has its meaning, so the meaning of a word is the combination of the meanings of 词素. 眼：a small hole, not deep / eye 睛：眼球. In fact, 睛 has a long history: You see, 睛 = 目 + 青。 Today 青 is a kind of color. But in ancient chinese, it means "精华" (essence) ----- the best part of a thing. 目 in ancient chinese means eye. an eye consists of orbit、eyelid、eyeball etc. So 睛 means the best part of eye. Obviously, it's not orbit、not eyelid, it's eyeball. So we use 睛 to express eyeball。 BUT, you can't use 睛 alone. We always use 睛 with 眼。 If you want to say eyeball, plz say 眼球, instead of 睛. Also, we can omit 睛, meaning that we can just say 眼 to express "eye". It's very hard to express every 词素 in every word. Because the contemporary Chinese has changed a lot but it developed from the ancient chinese. So if you want to know everything, you have to learn the ancient chinese and chinese etymology. This is very very very very hard. If you ask a chinese people the meaning of 睛, I believe he have no idea... So I suggest you learning chinese words and expressions, instead of 词素. For example, you just memorize that 眼睛 means eye, plz don't ask why and what is the meaning of 眼 and of 睛. When your chinese becomes very good, you can try to study each chinese word and their 词素. You see when we learn english, we just memorize the meaning of "correct", we don't ask the meaning of "cor" and of "rect". If we really want to know that, we must go to learn english etymology. So, you just need to do one thing: memorize that 眼睛 = eye. That's all. If you want, you can memorize 眼球、眼眶 together.

The meaning of 眼，睛 and 眼睛 is the same. But the usages of them are different. 1.We use 眼睛 or 眼 as the word "eye", but we seldem use 睛 . 【她的眼睛（眼）很漂亮。（CORRECT）】 【她的睛很漂亮。（WRONG）】 2.You can also find 眼 or 睛 in many other words like 左眼(left eye)，眼镜(glasses) and 目不转睛(staring) in which they have the same meaning as 眼睛。 【目不转眼（WRONG）】 Hope that can help.

眼，就是视觉器官。 睛，是眼珠。 成语"画龙点睛"，就是形容画家在龙眼上把眼珠点上，点了就生动起来了。意思是，能在说话或者文章关键处，简明点出要旨， 内容便马上变得生动有力。 成语"有眼无珠"， 说的是一个人没有好的判断力，像空有眼框而无眼珠。 可见"眼"不同"睛"，但"眼"和"眼睛"都是一般人所理解的视觉器官。

眼睛 is a complete word for "an eye" or "eyes", 眼 is a shortening of it

睛 ：eyeball 眼睛/眼：eye