Someone who doesn't have their own place and just crashes w/ friends/family. No permanent residence. They are technically homeless.
You see this a lot w/ families, which is why kids in poorer communities move in/out of schools so often. They crash somewhere for a few months, wear out their welcome or get on their feet, and move to another school district. It's awful for kids because there is no educational/curricular continuity and they end up behind and below grade level. By the time they get to HS, the fail out and/or drop out and the cycle continues.
You also see this a lot in rural communities. We think of homelessness as an urban problem, which it is for the loner, but in the rural communities, it looks a lot like what I described. I've known of families literally living in garden sheds or trailers w/ no running water and kids in elem, MS or HS in the Springfield metro. I'm sure it's a state-wide, nationwide issue.