zithern vs zither

zithern and zither both are nouns.

Word	Noun	Adjective	Verb	Adverb
zithern	Yes	No	No	No
zither	Yes	No	No	No

As nouns, zithern and zither are synonyms defined as:

zithern and 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

Other synonyms of zithern include cither.

zithern (noun)	zither (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 stringed instrument with strings stretched over a flat sounding board; it is laid flat and played with a plectrum and with fingers