The term "zimmer" is a part of
Star Trek lexicon that is not spoken onscreen but is instead used in
scripts. It's first use was in the script for the
Star Trek: Voyager episode "
Real Life" in which
The Doctor creates his own
holographic family.
So what does it mean?
In the days of holodeck use on Star Trek: The Next Generation whenever a holographic character is discontinued, said character simply faded from view into nothingness. Likewise, when a holographic character was activated, the character simply faded into existance. However, by the time of Star Trek: Voyager visual effects technology had improved, and seeing as how The Doctor was to be a holographic regular character, it was decided that he needed a unique method of entry and exit from scenes. The old fade-out technique was replaced with a brief visual distortion that depicts the holographic being taking form. The act of The Doctor (or other hologram) taking or losing form with the visual distortion is known as "zimmering", as in "The Doctor zimmers into Sickbay".
The word "zimmer" comes from the character of Dr. Lewis Zimmerman, the creator of The Doctor (and all other Emergency Medical Holograms that share his likeness). Since its debut on Voyager the zimmer effect has become the standard for holographic entries and exits on other Star Trek productions.
Reference:
Star Trek: Voyager Companion