Hologram

Holograms are those things, you know, that made the first ST story chronological appearance in ST:Enterprise, that mysteriously vanished throughout the whole of the ST:TOS thing, then reappeared as if brand new in ST:TNG as a derivative of the Volkswagen Transporter matter replication deelie, then worked completely differently somewhere in the middle of ST:Voyager using futons and horsefield technology, but was mentioned early on in Voyager as being VW Transporter matter replication technology again. 
Such a change in basic paradigm theory of Holographic technology was attributed to the scriptwriters strike of 2298, give or take a few thousand years.
The holodeck is a place that one of them hologram doolies got put there. Primarily used as sex parlours accomodating the vastly popular virtual Andorian slave girls program. No other holo-programs existed for decades except the ladies had a virtual Patrick Stewart with bare sweatty chest, including kung fu grip and eagle eyes, to play with.
During this period a great crisis ensued as food productivity and space exploration ground to a halt. Visits to holo-suites were up 20 million percent. Such a crisis was eventually subdued by the great scriptwriters return to work of 2298 point oh oh oh one, plus or minus a few thousand years. Wherein all the characters just went back to work for no reason.
Holograms also appeared out of small ashtray shaped devices. Such ashtrays would typically show images of dead people in extremely bad taste (See Tasha Yar), family photos and poorly rendered starsystems that looked very similar to very basic CGI and video editing techniques of the late 1980s and early 1990s. 
Something not portrayed on the screen due to watershed restrictions involved the little devices reproduction of naked Andorian slave girls or Patrick Stewart. The devices were very portable, could typically display holo images for many seconds at a time, and doubled up as ashtrays.
Here is a 3D representation image of your typical hologram image -
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As you can see, the likeness to that of a Germalian lemon is astounding.