Definition of middle position
Middle position means "middle position". Not everyone will agree on the specific seats that are the "middle position" seats. But basically middle position means that on a poker table, you are in the middle position. middle position will not be the same on a shorthanded table as it is on a full ring table. Also, the middle positions 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 to identify the positions
On a table of 9 players, the first school will identify the seats as 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 that a convention is that on a 10-player table, the positions are named as follows: 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 specific seats assigned to middle position (2 seats, MP1 and MP2).