(covers information from several alternate timelines)
A nacelle pylon, also known as a warp pylon or a nacelle strut, was a large support structure that connected a warp nacelle to the hull of a starship. A nacelle pylon housed the systems that directly linked a warp nacelle to a starship's warp core.
In an alternate 2263, the USS Enterprise was engaged in battle with Krall and his army of Swarm drones above Altamid. Immediately overwhelmed, they attempted to flee, but the drones managed to sever both nacelle pylons from the engineering hull before they could jump to warp. (Star Trek Beyond)
While attempting to break the warp 10 barrier in a holographic Cochrane in 2372, Tom Paris reported a fracture in the port nacelle pylon due to subspace torque. Later, after the problem had been corrected, Paris broke the barrier with both pylons secure. (VOY: "Threshold")
In an alternate timeline in 2374, USS Voyager's nacelle pylon began to buckle as a result of encountering a moving micrometeoroid field while operating without the use of its navigational deflector. (VOY: "Year of Hell, Part II")