A Christmas tree is typically, according to Human tradition, any type of Earth conifer tree that is decorated with lights and ornaments at Christmas time by those that observe that holiday. (TNG movie, novelization & comic adaptation: Generations, VOY episode: "Death Wish")
In 2269, Chief Engineer Montgomery Scott attempted to design a four dimensional Christmas tree, drawing up schematics during a Christmas party. He couldn't find any technicians or engineers who were interested in the design, and Spock likewise seemed uninterested. (TAS - Log Two novelization: The Survivor)
A Christmas tree with lights can sometimes resemble a damaged piece of technology, because of the random nature of the draped decoration. In 2285, Scott advised Admiral Kirk and the bridge not to give, "too many bumps" as the Enterprise engineering section (and its outbound systems) were bypassed like a Christmas tree, damaged by Khan and the hijacked USS Reliant. (TOS movie & Duty, Honor, Redemption novelization: The Wrath of Khan)
Captain Jean-Luc Picard once described a badly damaged Cardassian Galor-class warship as "sparkling like a Christmas tree", before its power failed and she started to drift. (TNG - The Dominion War novel: Behind Enemy Lines)
- TNG movie, novelization & comic adaptation: Generations
- VOY episode: "Death Wish"
- TAS - Log Two novelization: The Survivor
- TNG comic: "Spirit in the Sky!"
- TOS movie & Duty, Honor, Redemption novelization: The Wrath of Khan
- TNG - The Dominion War novel: Behind Enemy Lines