Lippage
Also called: tile lippage, uneven tile, tile lips
When adjacent tiles don't sit flush and one edge stands proud of its neighbor. The most visible flaw in a tile job, caused by an uneven wall, uneven adhesive, or tiles not pressed in consistently. Industry tolerance is about 1/32 inch for narrow joints.
Under-cabinet lights rake across a backsplash at a low angle, which makes lippage of even a millimeter cast a shadow line. Once the adhesive cures there’s no fixing it short of pulling the tile.
Causes, in order of frequency: a wall that isn’t flat (skim-coat humps and dips before tiling), adhesive combed unevenly or allowed to skin over, tiles of slightly different thickness (common with handmade-look ceramic and natural stone), and pressing some tiles harder than others. Large-format tiles and mosaics are most prone to it.
Prevention: check the wall with a straightedge, comb adhesive in one direction with the right notch size, set each tile with the same twist and pressure, and run a straightedge across every few tiles while the adhesive is still workable. Leveling clips exist for floors and large tiles but are overkill on a subway backsplash — a level and attention do the job.
The Tile Council of North America sets allowable lippage at 1/32 inch for grout joints under 1/4 inch — roughly the thickness of a credit card.