Rosin
Resin rubbed onto the bow hair of a string instrument.
What it means
Rosin is hardened tree resin rubbed onto the hair of a string player's bow. The friction this creates lets the bow grip and release the string in rapid succession, which is what actually sets the string vibrating.
Musical use
Without rosin the bow slides almost silently across the string; rosin increases the friction between horsehair and string enough to produce the stick-slip effect behind the characteristic bowed tone. Rosin comes in lighter and darker grades that differ in hardness and tackiness; lower string instruments such as double bass or cello typically use softer, darker rosin, while higher instruments such as the violin favor a harder, lighter grade.
Performance practice
Apply rosin regularly but sparingly before playing — too much produces a scratchy, dusty tone and leaves residue on the instrument's belly. Wipe excess rosin off the strings and the top of the instrument after playing, since it otherwise builds up.