Webster's English Dictionary

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apsis

ap.sis \'ap-s*s\ \-s*-.de-z\ n or ap.si.des [NL apsid-, apsis, fr. L, arch, 
   orbit, fr. Gk hapsid-, hap] pl sis, fr. haptein to fasten 1: the point in 
   an astronomical orbit at which the distance of the body fro m the center of 
   attraction is either greatest or least 2: APSE