Explanation：夢

 

夢(dream)

夢 means a series of events or images that occur in your mind while you are asleep. Its 部首 is "夕", which means "evening" or "sunset". 

This 字 has existed for a long time, as far back as the 甲骨文 (oracle bone script) period. 甲骨文 is the oldest recorded form of written Chinese. 

This is the original form of "夢". It is a person who is lying on the bed. Let me rotate it 90 degrees. This is a person, this is a bed.

According to the records, this word should evolve in this order. You can see that the last one is already very similar to the one we are using now.

Later it was classified under the 部首 "夕", probably because the ancients believed that sleep was mainly at night.

But some dreams can be had without the need for sleep. So we can classify "夢" into two types:

"a series of events or images that happen in your mind while you are asleep"

or "something you really want to happen, but sometimes it is not very likely".

Now, let's look at the words that can be combined with "夢" to form 詞.

詞

夢想、夢寐以求 (dream about/of something)  means to think about something you want very much, even in your sleep.

• 我夢想有天能在森林裡擁有一棟房子。
  I dream of having a house in the forest one day. 
  
• 那是我夢寐以求的工作
  That's my dream job.
  

Unrealistic imagination is called "幻想"(illusion)

• 我對我所嫁的男人不抱任何幻想。
  I'm under no illusions about the man I married.
  
  

The 詞 "白日夢" is usually translated as "daydream," but I think they have a slight difference.

According to the Cambridge Dictionary Online, "daydream" is  a series of pleasant thoughts about something you would prefer to be doing or something you would like to achieve in the future. But 白日夢 means that what you imagine in your mind is fantasy and won't come true.

• A: 她說不定也喜歡我！She might like me too!
  B: 做你的白日夢去吧！Yap! in your dreams.

夜長夢多

夜長 The night is long.

夣多 Dreams appear one by one.

The extended meaning of this word is that the longer you wait, the more chances there are for things to go wrong.

• 這份合約最好快點簽訂，以免夜長夢多。
  It's best to sign this contract as soon as possible, in order to avoid other things happening and causing changes in the plan.
  
  

Now let us look at the words related to our dreams while we sleep.

惡(噩)夢、夢魘(nightmare)

噩夢and 惡夢, both are correct, but the first is more common.

they can mean: 

"a very upsetting or frightening dream."

• 我真不該看那部電影的，整個晚上都在做惡夢。
  I shouldn't have seen that movie - I had nightmares all night.

or "an extremely unpleasant event or experience or possible event or experience".

• 我大學時期最大的夢魘就是考試不及格。
  Failing the exam was my biggest nightmare in college.
  

夢遊(sleepwalk) means to get out of bed and walk around while you are sleeping.

• 如果遇見夢遊的人，不要試圖叫醒他們。
  If you meet someone who is sleepwalking, don't try to wake them up.

There is a special phenomenon that occurs only during sleep.

託夢 means spirits and ghosts enter people's dreams to give messages or guidance.

• 這組樂透彩號碼，是我奶奶昨天托夢告訴我。
  This set of lottery numbers was told to me by my grandma in a dream yesterday.
  

美夢成真 dream come true

• 如果這組號碼是真的，那就美夢成真了。
  If this set of numbers wins, it will be a dream come true.