zither vs stringed instrument

zither and stringed instrument both are nouns.

Word	Noun	Adjective	Verb	Adverb
zither	Yes	No	No	No
stringed instrument	Yes	No	No	No

As nouns, stringed instrument is a hypernym of zither; that is, stringed instrument is a word with a broader meaning than zither:

zither: a musical stringed instrument with strings stretched over a flat sounding board; it is laid flat and played with a plectrum and with fingers
stringed instrument: a musical instrument in which taut strings provide the source of sound

zither (noun)	stringed instrument (noun)
a musical stringed instrument with strings stretched over a flat sounding board; it is laid flat and played with a plectrum and with fingers	a musical instrument in which taut strings provide the source of sound