A circumfixing construction is a °construction in which a °prefix and a °suffix occur on the same °root and occur always together without contributing separate meanings.
Note that it is not possible to say that "a circumfix is a kind of affix", because affixes are by definition morphs (i.e. their shape is a continuous sequence of segments). The term "a circumfix" can only be an abbreviation for a circumfixing construction, or the two affixes that figure in it.
"Circumfix: An affix that appears in two pieces simultaneously with a prefixal portion and a suffixed portion. The participle of the German verb machen (‘to make’) is ge-mach-t, with the circumfix ge ... t appearing on both sides of the stem mach." (Gebhardt 2023: xiv)