A cross section was a view of the inside of an object used for purposes of identifying a fault with the object.
In 2366, Geordi La Forge asked someone to bring up a cross section of computer core processor four five one. (TNG: "Evolution")
Later that year, La Forge asked USS Enterprise-D's computer to create a cross section image of the starship's dilithium crystal chamber. Needing a better view of the image, he then asked if a cross section image could be created on the holodeck. (TNG: "Booby Trap")
In 2367, Doctor Beverly Crusher wished to do cross sections of brain tissue from dead crewmembers of the USS Brattain. She later presented scans from a random cross section of Enterprise crew members. (TNG: "Night Terrors")
In 2370, Data showed the Enterprise's senior staff a cross section of his positronic net. (TNG: "Emergence")
In 2375, Seven of Nine combined information taken from tricorders, holo-images and astrometric scans to create a cross section of an artificial habitat appearing to contain members of various Alpha Quadrant species. (VOY: "In the Flesh")