Math operator is a symbol that represents a numerical operation.
Matrix is in the beginning and composition of metal type, a part of metal (This is usually brass matrix or copper matrix) used to cast type. A personality was either stamped or carved onto this metal.
Middle space is a fixed space equivalent to one-fourth the breadth of the em space. It is also known as the 4-to-em (4-em) space, or middle space.
Mirror is in computer-aided type design, to create a change to a personality that results in a reflected or mirrored image. It depends upon the software, different mirroring functions can be performed. This process is also known as flip.