difference between meanwhile and meantime

meanwhile - (The word ‘meanwhile’ is an adverb. It means ‘at the same time’. It is usually used at the beginning of the sentence and has a comma after it)
meantime - (The word ‘meantime’ is a noun. It is actually the noun of the adverb ‘meanwhile’. Since it’s a noun we will use it differently. It is usually used in a phrase like ‘in the meantime’ or ‘for the meantime’.)

What is the difference between meanwhile and meantime?

• ‘Meanwhile’ definition and use

  The word ‘meanwhile’ is an adverb. It means ‘at the same time’. It is usually used at the beginning of the sentence and has a comma after it. For example,
  I walked to school. Meanwhile, it started raining. (It was raining while I walked to school.)
  Jack worked all day. Meanwhile, his friends were relaxing. (Jacked worked, but his friends didn't at the same time.)
  
  

  ‘Meantime’ definition and use

  The word ‘meantime’ is a noun. It is actually the noun of the adverb ‘meanwhile’. Since it’s a noun we will use it differently. It is usually used in a phrase like ‘in the meantime’ or ‘for the meantime’. For example,
  I started cooking dinner. In the meantime, the guests arrived at the party. (I was cooking dinner while guests arrived at the party.)
  We don’t have money to fix the shower. You will have to take a bath for the meantime. (While the shower is broken, you will have to take a bath.)

• The main difference is that ‘meanwhile’ is an adverb and ‘meantime’ is a noun. The meaning of the words are the same. Some people will use ‘meanwhile’ as a noun but this is not common. If you want to be correct, always use ‘meantime’ in a phrase like ‘for the meantime’ or ‘in the meantime’. You can use ‘meanwhile’ by itself.

MEANTIME/MEANWHILE

• Although most authorities now consider these words interchangeable, some people still prefer to use “meanwhile” when it stands alone at the beginning of a sentence: “Meanwhile the dog buried the baby’s pacifier in the garden.” They prefer “meantime” to be used only in the expression “in the meantime”: “In the meantime, the dog chewed up my last tennis ball.”

• Although most authorities now consider these words interchangeable, some people still prefer to use “meanwhile” when it stands alone at the beginning of a sentence: “Meanwhile the dog buried the baby’s pacifier in the garden.” They prefer “meantime” to be used only in the expression “in the meantime”: “In the meantime, the dog chewed up my last tennis ball.”

• This is true, but meantime can absolutely NOT be used by itself at the beginning of a sentence, so it is not at all interchangeable with meanwhile in that case. You would never say:
  
  "Meantime, I was looking around for something to do". 
  
  "In the meantime" must be used in this case for the sentence to have any meaning.