volveré y seré millones
Hmm, good, practice for my oncoming DOOM dictionary/reference.
A conversion is when you transform DOOM's theme and looks into something else, like Batman, Aliens, some other movie, some comic, Hacx, characters and monsters you created, etc. The term was 1st used for Aliens TC (and is actually part of the name of the project) then other designers used it from there. Aliens TC was a "total" conversion because Fisher wanted to include everything he could from the movie, excluding anything about DOOM but not necessarily the artwork and sounds, if they fit in (the fact that DOOM's artwork and theme is in many respects aliens-like probably encouraged him, even.) As for "partial," this is vague and is simply an extension of the naming Fisher used for Aliens TC. It just means a certain part of DOOM's theme is changed, like making DOOM occur in a western scenario, for instance, or making a new set of enemies, but it's still appropiate to define it as "DOOM" even if the story is changed a whole lot. It's not really a 'serious' definition, as I see it, just a middling, vague term.
A megawad is simply a large WAD, with many maps, that is. The idgames definition seems to be something like 20 or more maps. That seems reasonable.
As for an episode you have DOOM's 9 level-sets, or WADs replacing one or more of these sets (sometimes not using up the secret level.) Some people say "mini-episode" when talking about a smaller set of maps. A DOOM II episode is defined by the sky used for it and the text appearing when it ends and is about 10 maps long, in WADs this term has been used for separate WADs with about 10 maps, Darkening for instance.
Warez All the Data?