An Arivida is the "mother" Aeternal who has made another of her kind through a special kind of intimate interaction. In the previous fiction, the result is called an Etone. Arivida is sometimes also the term used for an Aeternal Pledged who had been "reborn" into an Ovaform. Outsiders sometimes fear this process is a form of brainwashing, though ultimately it's little more than the Aeternal Pledged reaching out to someone who is hurt and offering both her heart and energy to them. This often manifests itself as another Pledged. "Her" is the word because so far the Arivida and Anina phenomenon had only been seen with the Aeternalae.
The bond shared between the Arivida and Anina is part of why the Aeternal Pledgeds' movement has been attractive to many individuals. It is a very unique form of love that is simultaneously reminiscient of a mother/daughter or sisters. It can even manifest itself in a form of Platonic love. The process the Pledged use right now, by far, is targetted almost exclusively to ease (and idealistically, completely halt) the pain of MtF Gender Dysphoric individuals who feel traditional science had "abandoned" them, and they're "invisible" to most spiritualities or religions. This is not to say, however, that the Touch is exclusively limited to healing gender dysphoria. Indeed, there are numerous cases, especially in the non-Mazoku Aeternalae where the Touch and creation of an Arivida-Anina relationship has no transgender implications at all, instead acting as a sort of malleable relationship, acting as mother, sister, or daughter, depending on the situation.
In the original fiction, Arividas creating Aninas were common place and very prolific in the attempt to help the Aeternalae achieve supremacy and even world domination. This has been played down a bit by the Pledged, however it remains important to our mythos.