A knot is a deliberate entanglement of rope or cord. The word "knot" covers all deliberate entanglements, and includes categories of knots including BENDS (which join (or bend) two ropes together) and HITCHES (which join a rope to a fixed object).
Knots are used in a wide variety of crafts such as canyoneering, climbing, fishing, rigging, weaving, sailing, construction, rafting, etc. Knot nomenclature varies widely between countries and crafts, with the same knot often having a different name in adjacent countries, even ones who nominally speak the same language. Knots will often have several different names even within a single craft in a single country.
Knots generally have a different name when they are tied in a different manner. Thus, an "overhand on a bight" is different than an "overhand on a bight via re-thread", though the result is identical.