Adler prints this unglossed headword as exostoa (e)cw/stoa), otherwise unattested; but it should surely be exostra (e)cw/stra), as in Hesychius (and Latin writers).
LSJ s.v.: 'stage-machine identified with the e)kku/klhma [see epsilon 699] by Hsch. and Poll.4.127, but distd. from it, ib.129'.
Hesychius cites Phrynichus (phi 762) for the term. On other senses see LSJ.