Major: a big, populous realm that has regular contact with Hybris and probably has a fair number of relatively stable portals (meaning as long as only a few people travel through them at a time, they'll stick around; sending an army through would break them).
Minor: a realm that is generally only lightly populated (and often quite small). It may still have regular contact with Hybris, but it probably doesn't have as many connections as a Major realm.
Lost: a realm which has no known connections with Hybris or the other realms. Occasionally folks from a lost realm will show up through a one-time portal, but often end up stranded. No one knows how many lost realms there are... a few scholars think that there are an infinite number, but most people scoff at that.
Sealed: a Minor or Major realm whose deity or deities have deliberately sealed it away, breaking all connections between it and other realms. The difference between a Lost realm and a Sealed one is that the Sealed realm has been deliberately cut off and would probably automatically drift back into contact if the inhabitants stopped working to keep it sealed.
Gathered: multiple realms can be drawn together until they actually form large connections, making them effectively a single realm... so long as sufficient magical energy is available to maintain these artificially strengthened connections. Such realms are referred to as having been "gathered" together.