A cylinder is a three-dimensional geometric shape roughly described as a closed ended tin can; circles on both end of a rectangle rolled end-to-end.
Cylindrical objects[]
In an alternate 1968, Captain John Christopher visually identified an UFO (specifically the USS Enterprise) as having "two cylindrical projections on top, one below [...] purpose undetermined," when identifying its nacelles and secondary hull." (TOS: "Tomorrow is Yesterday") Trip Tucker also described the nacelles as "the two large glowing cylinders [...] on the outside of the ship." (ENT: "Cold Front")
The Nomad probe encountered by the Enterprise in 2268 was described by Spock as being "roughly cylindrical" in shape, and "a fraction over one meter" in length. (TOS: "The Changeling")
In 2285, a sensor scan identified a Federation Mark VI photon torpedo casing as cylindrical in shape. (Star Trek III: The Search for Spock)
The Angosian escape pod detected by the USS Enterprise -D in 2366 was described as being "a cylindrical object approximately seven meters in length and three meters in diameter." (TNG: "The Hunted")
While instructing Julian Bashir to repair a subspace transmitter in 2370, he described one of the internal components as a "green cylinder." (DS9: "Armageddon Game")
In describing the 1936 Ford truck discovered by USS Voyager in 2371, Tom Paris explained to Harry Kim that it was driven by an "internal combustion engine, a reciprocating piston cylinder design." (VOY: "The 37's")
Odo also kept an infant Changeling in a glass cylinder in 2373, as he attempted to nurse it back to health. (DS9: "The Begotten")
The series 5 long-range tactical armor unit discovered by USS Voyager in 2375 was described by The Doctor as being "over a meter in length, cylindrical, [and] quite sleek." (VOY: "Warhead")