Bessel functions are a component of warp equations. They are used for the calculation of the energy needed for a warp jump.
In 2364, Starfleet propulsion expert Kosinski, when asked how the USS Enterprise-D flew to the Triangulum Galaxy, he stated in his long, windy explanation that he may not have applied the Bessel functions correctly. In the opinion of Commander William Riker, the explantion was nonsensical, yet he couldn't provide an explantion for how the starship arrived at their current position. (TNG: "Where No One Has Gone Before")