What did "Boarder" mean in the 1850 Census? I'm guessing it might include anyone not a wife, son, or daughter.

I’m working in a frontier community with 171 people – 73 of which are “boarders.” Quite often they are living with families. Of the 29 households, 2 consist solely of single men, mostly fishermen in this Great Lakes community. The other 27 households contain at least one direct relation.

No person is identified who is not a Head, Wife, Son, or Daughter – except for “Boarder.” I would have thought there’d be a brother, brother-in-law, housekeeper, mother. My population is small and the community in early settlement days.

What instructions were given to the Census enumerators.

You are correct. Boarders are non-relatives who otherwise have a non-specified relationship with the household head. This US Census Bureau paper seems to offer a nice discussion of who these people are. In this paper, boarders are defined as: “persons who live in the household of the householder and make cash or non-cash payments (e.g. chores) for their living accommodations.”