Webster's English Dictionary

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quadrilateral

1. quad.ri.lat.er.al \.kwa:d-r*-'lat-*-r*l, -'la-tr*l\ aj [prob. fr. 
   (assumed) NL quadrilateralis, fr. L quadrilaterus, fr. (Xquadri- + later-, 
   latus side : having four sides 
2. quadrilateral n : a polygon of four sides