An AutoCAD external reference. An xref is a variation on a block. A block is a collection of geometry that is identified by a unique name, is stored in the AutoCAD symbol table, and essentially behaves as if it is a single object. Xrefs share block characteristics, and they are similarly defined in the symbol table. However, unlike blocks, the geometry associated with an xref definition is not stored in the current AutoCAD drawing; it is stored in another AutoCAD drawing file. Like a block, there can be many instances of an xref in an AutoCAD drawing, but only one definition.
Typically, you use an xref to display the geometry of a common base drawing in the current AutoCAD drawing without expanding its size. This allows changes to the reference drawing to be reflected in any host AutoCAD drawings that refer to it.