College Trio: Collective name to refer to Vlad, Jack, and Maddie.
CW: Because sometimes "Clockwork" is just too long to bother with.
GW: Because sometimes "Ghost Writer" is just too long to bother with. (And his initials are written on the pediment of his mansion).
Frightmare: Nocturne's temporary name prior to the episode "Frightmare" airing.
Seņor Seximus: Nickname used by English-speaking fans for Vlad in the Spanish language version of the show.
Dan Phantom: A name commonly used for Danny's evil alternate-future self. Only originated in the video game adaptation.
Dark Danny: Another name used for Danny's evil alternate future-self. Shown during the episode's credits.
Dan Plasmius: Also Dark Danny.
Jack Plasmius: Halfa Jack in "Masters Of All Time".
Master of Longwinded Introductions: Technus.
Flip Flop of God: So "ghosts" are really just monsters from another dimension who happen to use ectoplasm? Okay, but what about the countless references to ghosts being dead or previously being alive? Or, say, the Dairy King.
Another example is Ember McLain. Her actions, language and attitude suggest she's a teenager with a desire to rebel against adults, which might be the reason she was getting constant shipping with Danny from fans. However, the idea of them having this kind of "relationship" didn't occur when admins on Hartman's forums stated she was much older, around her early 20's... which technically makes her an adult. Suddenly she's Skulker's girlfriend... and Skulker himself is an adult.
Lying Creator: Butch Hartman has said the ghosts are not meant to be spirits of the dead, despite many of the ghosts in the show are shown to once have been alive (Pointdexter, Desiree, the Dairy King, the Lunch Lady, Ember). See Flip Flop of God.
A Part Time Hero named Fenton. Also, Tucker shares the same last name with Richie Foley from Static Shock who is also a nerdish technologically obsessed side kick who does scheduling for the protagonist.
Danny was supposed to be a teenaged Ghostbuster on a motorcycle with a pet owl and a psychic link to Sam. Then the idea was tweaked into what it is now.
This could apply to the entire third season, as Steve Marmel allegedly wanted to get Darker and Edgier, following along the lines of "The Ultimate Enemy". There was even a fan project on imagining what it would be like, only it never got off the ground because they couldn't get details from Mr. Marmel. Marmel won't comment on what Season 3 was going to be about or why he was let go, mostly out of respect for his former partner Hartman.
According to Word of God, Danielle was meant to be revealed to the Fenton family and become Danny's little sister. Yet another plotline that was abandoned when the series was abruptly canceled.
The series was originally meant to end with a final battle between Danny and Dark Danny. One could argue it would have made a greater series finale than Phantom Planet.
Sam and Danny were originally meant to share a psychic connection before Butch Hartman decided against it, saying only Danny should have the special powers.
According to writer Steve Marmel, an original Running Gag would be that Vlad's manor would blow up every time.
The name "Vlad Plasmius" was claimed by Marmel to be written on a brick he bought.
Tucker Foley was named after Chris Tucker and Eddie Murphy's character Axel Foley from Beverly Hill Cops. Jazz was named after a character from the Next Men series. Vlad is derived from Vlad Dracula III, the original inspiration for Dracula.
For that matter, Vlad was originally intended to be a vampire, hence why his ghostly form heavily resembles one.
Also, hence the name "Plasmius"—from "plasma," or blood.
Danny Phantom's eyebrows were at one time (in production) white like his hair, but they were changed back to black—they made him look too old.
Look at Sam's room right after the theme song in "Control Freaks." Sam has a (musical) keyboard lying on her floor. Perhaps she plays!
Also, there's a red version of Ember's guitar in Jazz's room in "Fanning the Flames".
In a first season episode, where Tucker is editing his grades, the only ones he has A's in are Computers and ...Sewing.
According to one of the admins on Butch Hartman's forums (before the site eventually ended), Hartman stated not everyone can become a half ghost the same way Danny and Vlad did. Meaning if it were Sam or Tucker who entered the portal, it would have more likely killed them than turn them half ghost.