Family (US Census)
A family or family household is defined by the United States Census Bureau for statistical purposes as "a householder and one or more other people related to the householder by birth, marriage, or adoption. They do not include same-sex married couples even if the marriage was performed in a state issuing marriage certificates for same-sex couples. Same-sex couple households are included in the family households category if there is at least one additional person related to the householder by birth or adoption. Same-sex couple households with no relatives of the householder present are tabulated in nonfamily households."
Hence, households consisting of grandparents and grandchildren without the intervening generation are not "families", nor are same-sex married couples even where such marriages are recognized, nor unmarried opposite-sex partners (POSSLQs).
- Census report DEC_10_DP_DPDP1, available on-line [factfinder2.census.gov here].