"Ghost Envy" is a term coined by Jazz Fenton to describe a the feelings of jealously, anger, anguish, or general "envy" that humans may have in pertinence to ghosts or half-ghost beings. This feeling is temporary, but can last anywhere between a few hours to many years depending on the individual(s) and their circumstances (like who is envious of which ghost or half-ghost, what their personal relationship is like, their personal motives and moralities, etc.).
Jazz first uses this term in the Season Two episode, "Reality Trip," in which she explains to Danny that she is going to spend her summer working on a college entry essay about it. Despite his disinterest, this term later proves useful when Danny engages in a final showdown with Freakshow, who had been in control of the Reality Gauntlet to warp and manipulate reality to his advantage.
Before then, Jazz and Freakshow (while waiting for Danny, Sam, and Tucker to arrive) had a conversation about his subconscious issues with ghosts. When she witnessed his reaction to Lydia phasing through a wall being negative, she mentioned also having some jealousy over her little brother because of his hero status, being half-ghost, etc. This leads to the start of a connection when she admits that they both seem to have ghost envy (but to her advantage, Freakshow's case is more severe).
During the showdown between him and Danny Phantom, she tells her brother that Freakshow has ghost envy, so he needs to apply psychology to defeat him; thus, Danny starts showboating about his ghostly abilities. Freakshow becomes overwhelmed with envy as the boy steals his audience's attention and uses the reality gauntlet to turn himself into the biggest, baddest ghost imaginable -- at which point, Danny was able to trap him in a Fenton Thermos and take control of the gauntlet.
Later in "Forever Phantom," the faceless ghost Amorpho exhibits a mild case of ghost envy over Danny Phantom for his ability to change between human and ghost forms and the fact that his heroic status overshadows the latter's attention-seeking antics. This leads to a brief rivalry between the two until they directly talk, which reveals they were simply misunderstanding one another. They leave off with a truce based on mutual respect, although it is implied that Danny may (or may not) see Amorpho again.
Within the Danny Phantom: Spectral Connections series, the subject of ghost envy occurs for at least several human characters throughout the series progression. Pending more...