Definition of middle position
Middle position means "middle position". Not everyone will agree on the exact seats that are the "middle position" seats. But basically, middle position means that on a poker table, you are in the middle position. The middle position will not be the same on a shorthanded table as on a full ring table. Also, the middle position seats will not be the same if there are 9 or 10 players at the table. There are also 2 schools to indicate the seats (new school vs. old school). More details in the next paragraph.
The two schools for identifying positions
On a table of 9 players, the first school will identify the seats as follows: UTG, UTG+1, MP1, MP2, Hijack, Cutoff, Button, Small Blind, Big Blind. Another school will identify the seats as follows: UTG, UTG+1, UTG+2, Lowjack, HiJack, Cutoff, Button, Small Blind, Big blind. If the table has 10 seats, it would be according to the schools. But let's say it's a convention that on a 10-player table, we name the positions like this: UTG, UTG+1, MP1, MP2, Lowjack, Hijack, Cutoff, Button, Small Blind, Big Blind. See the image at the bottom of this paragraph to see the exact seats assigned to middle position (2 seats, MP1 and MP2).