(of marriages) of a marriage between one of royal or noble birth and one of lower rank; valid but with the understanding that the rank of the inferior remains unchanged and offspring do not succeed to titles or property of the superior WordNet 3.0 & Open English WordNet

Designating a marriage (or the wife involved) between a man of higher rank and a woman of lower rank, often having various legal repercussions (typically that such a wife has no claim on the husband's possessions or title). It was not an aspect of English law, but was common in other royal houses, especially in Germany. Wiktionary