However, its origin predates modern zoological nomenclature and derives from a term used by Aristotle to refer to "small fishes that bury... like the gudgeon.

They range in adult length from the 23 mm (1 in) miniature eel-loach, Pangio longimanus, to the 50 cm (20 in) imperial flower loach, Leptobotia elongata, with the latter weighing up to 3 kg (6.6 lbs).

[6][7][8] Most loaches are small, narrow-bodied and elongate, with minute cycloid scales that are often embedded under the skin, patterns of brown-to-black pigment along the dorsal surface and sides, and three or more pairs of whisker-like barbels at the mouth.

[9] The type species of the family Cobitidae, Cobitis taenia, has a body shape and pigment pattern typical of Cobitoidei.

The order is commonly known as "minnows, carps, loaches, and relatives," and has included the suckers (Catostomidae) and algae eaters (Gyrinocheilidae), these are now regarded as separate suborders, the Gyrinocheiloidei and the Catostomoidei.

[17][18][19] Owing to shared morphological characteristics (see osteology, below) the relationship of the botiid and cobitid loaches was particularly difficult to resolve until the advent of molecular phylogenetics.

[11] Loaches are found in a wide variety of habitats throughout Europe, northern Africa, and central and Southeast Asia.