A model's UVs say where each corner of each face lands on a texture: two numbers, U across and V up. Together they're the UV layout, also called the UV map.
The square where U and V run from 0 to 1, the whole texture once over. UVs outside it repeat the texture. The UV Layout draws the square with its grid and rulers.
A piece of the UV layout whose faces all join up: cut free from the rest along seams. Islands is one of the four selection modes.
An edge where the UV layout is cut, so the faces either side of it can lie apart. Split and Weld make and close them.
Laying a model's faces out flat, cut along its seams, as Flatten does. Generate UVs picks the seams and flattens in one go.
How far a face's UVs are from its true shape on the model. With no stretch, a texture's squares stay square on it. The Stretch preview colors it, and Relax evens it out.
How much of the texture a face gets for its size on the model. Even density spreads a texture's pixels evenly over the model: see Normalize Scale.
A UV vertex held where it is, so Flatten and Relax lay out the rest around it. See Pins.
Fitting the islands together in the 0-1 square, none overlapping, with a gap between: see Pack.
Islands kept together, to move as one and pack as one piece. See Island groups.
What a model's faces are made of: a color, and usually a texture. A model's faces are split between its materials. See Materials and textures.
The image laid over a model through its UVs.
An image saying how see-through each part of a texture is: white solid, black clear. See Opacity maps and color keys.
One color in a texture shown clear, so the parts it covers see through. In an .i3d model, a "!" in a material's name turns it on.
A run of edges, each leading straight on to the next across the faces between. Double-click an edge in Edges mode to select its loop.
A .3ds or .i3d file is built of chunks, each a piece of the model: a mesh, its faces, a material, the animation. The Source view lists them.
The part of a .3ds or .i3d file that places its objects and animates them: their hierarchy, pivots and keys over time.