Texture seam
The visible join where two UV islands meet on a texture — colour or light bleeds across the edge and shows up as a line on the finished model.
When a 3D model is unwrapped into a two-dimensional UV map, its surface is cut into islands. At the islands' edges the texture samples pixels that really belong to another part of the model — and at low mip levels they blur together into a visible seam.
The standard remedy is padding: you dilate the colour a few pixels around each island so sampling never reaches empty background. Lightmaps suffer the same problem in their lighting data and need either margins or stitched seams. A careful UV layout hides the rest by placing seams where they show least.