In old Vienna households, the term mezzanine was used for a floor in between the ground floor and the first floor. So it is “something in-between”.
In the financial world, the term refers to a tool that strengthens your equity base, which is lent in all its forms as a loan. Mezzanine capital is accepted as equity but co-determination rights of investors are excluded. For this reason, it is also more expensive than regular borrowed capital. It is simply “something in-between”.