Where two women, sans any male help, live together in a relationship, that does not necessarily involve sex.
Refers to where two women are living together, independent of any male support.
It was used to differentiate from those woman who were Lesbians, and living together as part of a sexual relationship.
Coined after the 1st World War, as there were many single woman left to cope, and to avoid being labeled as Lesbians, this term evolved to describe where two women joined forces, under one roof.
Many claim it was after the book by Henry James was released, that the phrase became accepted. This is due to the book detailing such a marriage, or situation.
Noteworthy: The play by David Mamet (Boston Marriage) in 1999 included explicit sexual relations, and now as Marriage is legal in the State, where Boston is part of, a Boston Marriage can become a legal entanglement, if desired.