Someone who is not officially a wife or husband but is considered to be one because she or he has been living with their partner for a long time(Cambridge Dictionary)
The person of the opposite sex or of the same sex who had been living in a conjugal relationship with you at least 12 consecutive months… was living in a conjugal relationship with you and was biological or adoptive parent (regally or in fact) of a child of whom you were also the parent. (Canadian Tax Act)