As a general term it is of course, also used for all sorts of things, i.e. jewellery, furniture, silver, printing, even, believe it or not, burial mounds.....
In overall terminological terms, it is, as Glen says, something that is created so that the image is below the level of the surrounding material.
the word is Italian of course, and means "incised"
I thought this a good description from
http://seeing.nypl.org/intaglio.html regarding the use of the term in printing:
Intaglio (Italian, "to incise") includes engraving, etching, and mezzotint, among other techniques. Reversing the relief process, in intaglio the artist cuts the lines to be printed, rather than cutting away the nonprinting surfaces. Although it is an ancient process, intaglio did not come into use in Europe for printing illustrations until the fifteenth century. Engraving allowed the scientific or medical artist to create a more precise and detailed line in a metal plate — copper at first, but later steel — than was possible in relief. Intaglio printing requires much more pressure than relief, since the ink is held in recessed grooves instead of on the surface of the plate, and so illustrations could not be printed on the same press as the text.