UV unwrapping
Flattening a 3D model's surface onto a plane so a texture image can be painted onto it.
A mesh lives in three dimensions, but the image wrapped around it is flat, so every vertex needs a 2D coordinate — a UV — saying where on the texture it samples. Producing that mapping means cutting the surface along seams and laying the pieces out like a skinned hide, minimising both stretching and wasted space. The letters U and V simply name the texture axes, picked because X, Y and Z were taken. Done carelessly it causes visible distortion and seams; done well it packs tightly, which feeds straight into texture atlases and lightmaps. Tools like Blender automate much of it, but game artists still tune seams by hand.