A Christmas tree
View of a Christmas tree from Voyager's viewscreen
A Christmas tree was typically any conifer tree that was decorated with lights and ornaments during the Christmas season.
While attempting to decipher cybernetic circuits implanted into Enterprise NX-01's systems in 2153, Charles Tucker suddenly witnessed them active and described is as having "just lit up like a Christmas tree." (ENT: "Regeneration")
After the first engagement with the USS Reliant, and prior to the battle of the Mutara Nebula in 2285, the hasty repairs made by Montgomery Scott to the energizers aboard the USS Enterprise were described as being "bypassed like a Christmas tree." (Star Trek II: The Wrath of Khan)
In 2372, the crew of USS Voyager found themselves tethered to some kind of large plant, later revealed to be a Christmas tree, after the Q who was later referred to as Quinn reduced the ship to the size of a Christmas ornament, as he attempted to escape from Q. (VOY: "Death Wish")
When Captain Jean-Luc Picard was in the Nexus, he found himself celebrating Christmas with a large family, and one of the holiday decorations was a Christmas tree. (Star Trek Generations)
In 2373, one of the Swarm ships fired a polaron burst at Voyager and illuminated their shield polarity which attracted the attention of thousands of Swarm ships in their space. Chakotay then said "As far as anyone's sensors are concerned, we're lit up like a Christmas tree." (VOY: "The Swarm")