I think the correct term is mid surface.
Often when larger companies where published geometry is more prevalent, part
designers are required to publish 'a-surface', a 'b-surface' and a
'c-surface' often called a mid surface for those FEA guys. Often times the
c-surface or mid surface can be used with top down design to create cuts for
ribs that will be placed into a part. The mid surface is offset half way
between the a and b surfaces.... Which is why mid surface makes for a
greate cutting tool for those ribs. Never a coincident error when part
accuracies are different.