Literally an “in-between rental”. Someone holds a flat on a normal German lease, leaves for three months to travel, do a residency, or work abroad, and rents their place to you while they are gone. They stay the tenant; you are there on a side agreement with them.
This is the market most newcomers to Berlin actually land in, and it exists because the permanent market is effectively closed to someone with no German payslip, no SCHUFA record and no address. A Zwischenmiete asks for none of those. What it asks for instead is speed: the good ones are gone in hours, because the person leaving has posted once, to a group of ten thousand people, and will take the first reply that reads like a human being.
The catch is that your rights are only as good as your paperwork. A Zwischenmiete is a real tenancy under German law, but if the arrangement is informal you will struggle to prove it, and two things you will want later depend on it: the Wohnungsgeberbestätigung you need to register your address, and the deposit you want back. Get the agreement in writing, even a short one, and check the tenant actually has permission to sublet.
Note the word is used loosely in ads. A “Zwischenmiete” is often a room in a shared flat rather than a whole place, and is sometimes an open-ended arrangement being advertised as temporary because temporary is what people search for.