M - Commands   
M2P (command)
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It sets the midpoint between two points

 Snap toolbar: 

  Contextual menu:   Mid between two points...

 Command line: M2P

The command is only enabled while inserting or modifying objects, when prompted to specify a point.
It is  possible to use the MTP command modifier with active Object Snap or with the local Object Snap modifications.

At any prompt asking for a point, enter M2P.

Specify first point: Specify a point

Specify second point: Specify a second point