A knife is an instrument that has a sharp blade attached to a handle and is used for cutting. Typically a knife is single-edged, if both edges of the knife blade are sharped, it is usually considered a dagger. Everyone would have worn a knife in period, if only for eating.
In England all knives before the mid-14th century had whittle tangs, but by the early part of the 15th century the scale tang had become the dominant type.
Certain styles of knives are strongly identified with certain cultures or time periods, for example the migration period seax, the Russian kindjal or the Arabian jambiya.
Knives in the SCA
Remember, if you draw steel at an SCA event, you must say "Clear!" to warn others.