For me,
imaging is a general, umbrella term that covers how a track's soundscape is made to look by an IEM. Holography is one of those aspects, and so is soundstage size, instrument placement, imaging accuracy - how tight or focused a note looks in the stage - etc. My definition of holography is similar to
@Tristy's, where it's how 3D a note looks in the image. I'd also add that it's related to the IEM's background blackness as well. The blacker the IEM's background, the more the note can stand out from it, making it seem more 3D. It's like how a TV panel with minimal backlight bleed and super-dark blacks will make images punch and pop more. The same goes for a stage that expands out-of-head. That'll help sell the illusion that the song isn't just happening in your head. So, all those things contribute to palpable holography to me, which in turn adds to an IEM's imaging.