zenith
[astronomy] In astronomy, the point on the celestial sphere directly above an observer. Both the zenith and nadir lie on […]
GIS Dictionary
zenith angle
[ESRI software] In Survey Analyst for field measurements, a vertical angle that is formed by the intersection of two lines
zero length line event
[linear referencing] In linear referencing, a line event whose from-measure is equal to its to-measure. A zero length line even