EQUISETUM, or HORSE-TAIL, in botany, a genus of the cryptogamia filices class. The fructification is disposed on an oblong spike, and of an orbicular figure. There are seven species, six of which are natives of Britain, viz. the sylvaticum, or wood horse-tail; the arvense, or corn horse-tail; the palustre, or marsh horse-tail; the fluviale, or river horse-tail; the limosum, or smooth horse-tail; and the hyemale, or rough horse-tail.