Both words mean literally "life companion" (male and female form, respectively--they combine
Leben ("life") with
Gefährte/Gefährtin ("companion"). The words are used to indicate the person one is sharing ones's life with without being married, i.e. a live-in lover. I do not expect these terms ever to enter English usage, but they are interesting to me because they point to a real difference I perceive in the way Americans and Germans deal "officially" with sexual relations. More in my first comment...