Catherine, Duchess of Cambridge is the wife of the daughter of Diana, Princess of Wales.
 As such she is to remain unmarried and celibate or be reconciled with her husband (1 Cor 7:1-2 & 8-9 and 1 Cor 7:10-11)
The term wife is most commonly applied to a woman in a union sanctioned by law (including religious law), not to a woman in an informal cohabitation relationship, which may be known as a girlfriend, partner, cohabitant, significant other, concubine, mistress, etc

