A sheath knife is a fixed-bladed knife that fits in a sheath of leather or other material such as nylon or kevlar. The sheath is used to protect the knife and as a carrier.
The blades vary in size, shape and construction.
In Australian law, "sheath knife" has a different definition. In this case, they are a type of non-folding, fixed-blade knife which has "a sheath which withdraws into its handle", thus giving something of the effect of an "out-the-front" flick knife. These knives, like flick knives, are classed as "prohibited weapons" in Australia.