Hi and welcome to Android Forums!
MID stands for Mobile Internet Device, and is actually a fairly general term. However, I personally categorize tablets separately: the standard tablet will almost always have wifi built in, and able to access the Internet through an available wifi connection.
MID tablets, on the other hand, would not need wifi access, as they use cellular data from a wireless provider like Verizon, Sprint or T-Mobile. They are usually sold directly from the provider; whereas standard tablets are available everywhere.
I hope this helps.
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