1. A reference to having a head of medium breadth, with a cephalic index between 76 and 80.

The cephalic index is a rating scale which is used to measure the size of the head.

The rating on the cephalic index is obtained by multiplying the maximum width of the head by 100 and dividing that number by the maximum length of the head and the numbers between 75 and 80 are considered normal.

2. Having the cranial cavity of medium capacity which is neither megacephalic (abnormally large head) nor microcephalic (abnormally small head).
3. Etymology: from the Greek word mesos, "middle" + the Greek word kephale, "head" + Greek ikos, "pertaining to". Put the words together and you have "pertaining to (a) medium head."