Quick HP fandom question - Severitus/ Sevitus

At first, I though it was just an alternate spelling, but now I'm not sure. As I understand it, "Severitus" is a HP fandom lingo for a story in which Snape is somehow Harry's father - biological or adoptive. Right? So what's "Sevitus"? Are they distinct words with different meanings (one biological, one adoption/ mentor fic)? Help me out here, HP fans. I really did try to google this, but I'm not getting anything particularly useful.

ETA: So, consenus is, "Severitus" was originally a particular challenge, not unlike the "Marriage Law" Challenge, and it's been somewhat appropriated to cover fics that share some of the same characteristics as those in the challenge. The name comes from the originator of the challenge. "Sevitus" denotes fics that deal more generally with Snape as father/father figure, but many authors use "Severitus" interchangeably with it.

These are hilarious

Seen in hogwarts_today: prompt claims open for unusuallygen. A listing of pretty much every 'ship plot you can think of (and some I'd never even considered) where the twist is that the expected thing, hot (monkey/wizard/vulcan) sex does Not happen, the (guy/girl/wizard/vulcan) does Not get the (girl/guy/witch/captain)....

They're a riot to read through, and most are applicable to any fandom at all. There are a few specific to Buffy/Angel, Dr. Who, House, and other fandoms I've seen mentioned on the f-list. Enjoy.