1) to combine art elements (type, illustrations, lines, photographs, etc.) into a unified whole, using manual tools and wax/adhesives or using a computerized system to manipulate and combine the art elements into a whole page. 2) same as "mechanical" - the term for a camera-ready piece of artwork, including type, photos, line art, all attached to one sheet or matte board.
Traditionally, the process of assembling mechanicals by pasting galleys and line art in place. In desktop publishing, traditional pasteup has largely been replaced by electronic page assembly.