a disk with a sector removed that when rotated in front of a light source allows the periodic passage of flashes of light: used in studying the motion of a body.
Also, e⋅pis⋅ko⋅tist⋅er.
Origin: 1900–05; < Gk episkot(ízein) to overshadow (see epi-, scotia, -ize) + -ist+ -er1
[uh-pis-kuh-tis-ter, ep-i-skuh-tis-ter] 
kəˌtɪs