A marriage between persons one of whom is a member of the Catholic Church and the other a member of some Protestant denomination.
The phrase is the equivalent of the kitchen Latin, mixta religio (mixed, or mixture of, religion).
It is sometimes used to mean the union in marriage of a Catholic with one who is not baptized but this is expressed better by the phrase disparitas cultus (difference, or inconsistency, of worship).
Since differences of belief in religious matters very frequently occasion incompatibility in domestic relations, impediments to the proper religious observance of both partIes, and differences over the religious training of the children, the Church, to safeguard the religion of the Catholic party, requires that the marriage take place before a priest only, that the one who is not Catholic will not interfere with the religious observances of the other nor with the training of the children as Catholics.
See als, disparity of worship.
New Catholic Dictionary