Hmmm.  My understanding is that "zither" covers them all.  Some of
the other names derive from a common term:  zither, cytherium, cithera,
cittern, gittern, guitar, sitar, kithara, chitarra, and on.

From "Musical Instruments of the World" we read on page 164:

"Chordophones are instruments in which sound is made by the vibration
of strings.  There are five basic types: bows, lyres, harps, lutes,
and zithers. ... Zithers appear in a wide variety of styles, ranging
from simple tube zithers to the sophisticated keyboard instruments of
Western Europe."

Zithers are distinguished by having strings raised by bridges, which
run along the instrument's entire length parallel to the body.  The
resonator generally comprises the entire body, but some have necks.
They are distinguished from harps by use of a bridge, and strings
parallel to the instrument body.  Harp strings attach directly to the
resonator and run at an oblique angle to the neck. (Auto-harp is a
trade name.  It's actually an auto-zither!)  Zithers are distinguished
from lutes by having a flat back (i.e., a guitar is a zither rather
than a lute).

The current (generally accepted) taxonomy, derived from the system
published in 1914 by Erich von Hornbostel and Curt Sachs, categorizes
instruments according to the way in which sound is produced.  The
zither category includes:  pianoforte, clavichord, hammered
dulcimer, spinet (bentside), virginal, harpsichord, psaltery, cittern,
guitar, ukulele, Appalachian dulcimer, auto-harp, and a whole plethora
of board, long, simple, tube, and stick zithers found in nearly every
culture, in endless variation.  Further inventions, developments,
hybridizations, etc., continue to produce ever more permutations.

Regards,

Robert Linnstaedt

 [ Thanks, Robert.  It seems that we need a family tree to differentiate
 [ all these instruments of the zither family.  If we adopted Latin
 [ names, like those created by biologists, we might have
 [
 [   zithera guitara acoustica : the acoustic guitar
 [   zithera zithera zithera : the Austro-Hungarian zither
 [   zithera zithera conchordia : the zitheroid with chords
 [
 [ (I chose feminine words because, to me, a guitar is a lady!  ;)
 [ -- Robbie