A continuum is a timeline of sorts. It is somewhat more abstract, however, from timelines common in First Realms, as those often split and become separate First Realms. Instead, continuums primarily differ as completely separate multiverses, sets of First, Second, and Third Realms, if such realms even exist in other continuums.
Continuums typically differ in one major metaphysical respect from each other. While First Realms may differ because of a singular choice in time (such as you choosing whether or not to click on the link that brought you here), continuums tend to differ in ways that either drastically alter the nature of spacetime and the combination of realms, or that considerably alter metaphysical constants, such as the nature of Gods, Eris, or, when dealing with the study of Aeternalae, the Alphariginae.
In Aeternalae Mythos, ten primary continuums exist with ten different Alphariginae and ten different methods by which the Aeternalae were created, none any less true than another. However, little is known about these different continuums, and the methods of travel between them is completely unknown (though some speculate that the Second or Third Realm has the thinnest barriers between these other continuums). What little we know of these other continuums is known from anecdotal evidence from the few pieces of extraneous knowledge collected in First Realms hinting at other continuums. The fact that many of these anecdotes arrive in the form of little known legends carried down orally leads some to believe that perhaps the Second Realm is either common between the continuums, or contains the method of travel between these continuums, if not at least the thinnest barriers between these continuums.
Of the ten primary Aeternal continuums, the one in which we live is the Leena Continuum, and the only other one we know of is the Meriax Continuum. Recent anecdotal evidence may have revealed a third, but its existence is not known definitively.