Extraordinary photos set 'The Pond' apart; The Pond, by Gerald Thompson and Jennifer Coldrey. Photographs by George Bernard. Cambridge, Mass: The MIT Press. 256 pp. $25.

JUST about the only thing in ''The Pond'' not described with great precision is the body of water itself. ''There is no single feature that can be used to distinguish a pond from a lake or any other body of freshwater,'' say Gerald Thompson and Jennifer Coldrey.

''The best we can do is to use a combination of characters, and define a pond as a rather small, shallow body of freshwater in which there is little difference in temperature between the surface and the bottom.''Authors' itals

In any case, Thompson and Coldrey take us through a pond's various zones (swamp, floating-leaf, and emergent, submerged plant, algae, fungi, bacteria). They discuss animals in and near the pond, ranging in size from protozoa to the vertebrates. Their emphasis, though, is on microscopic to small. Finally, they provide a glossary and classification tables.

What sets ''The Pond'' apart from other books of its type are George Bernard's color photographs, which when not merely very good are usually extraordinary, as in the case of the 15-shot sequence of a dragonfly emerging from nymph to adult.

Although no one could go wrong simply reading ''The Pond'' straight through, it is also a useful cross between a field guide and a reference volume - not detailed enough to be the former or so massive or intimidating as the latter.

However one uses it, this is a first-rate piece of work, from which much can be learned. Numerous are the pleasures of the text and photographs.