THE WORLD around us is gray and cloudy with imprecision: The thingamajig is connected to the doodad next to the hickey on top of the whatchamacallit. When we look at, say, a wave, we see just a wave--though a surfer or oceanographer might notice the scend, crest, spindrift, curl, shoulder, trough and back. With this terminology even something so amorphous and fluid as a heap of water can be exactly described.

What's What--my pick for the reference book of the Christmas season--brings the fuzzy into focus through show-and-tell. Each page of this "visual glossary" carries an annotated photograph, drawing or schematic naming the physical characteristics of animals, the details of wearing apparel, the nuts-and-bolts of machinery, the features of the earth around us, or the materials and methods of art. A necktie possesses at least 10 terminologically distinct parts; a steam locomotive several dozen. Because the illustrations are categorized, indexed and minutely cross-referenced, one can readily discover the real right name of any ordinary thingamajig.

A useful reference for anyone, What's What should appeal especially to browsers who love words, technical jargon, and the odd fact. Where else could one find a table of the symbols used by hobos? Or the names for the frills and bows on the uniforms of French maids?