quadrilateral (n.) Look up quadrilateral at Dictionary.com
"four-sided," 1640s, with -al (1) + Latin quadrilaterus, from quadri- "four" (from PIE root *kwetwer- "four") + latus (genitive lateris) "the side, flank" (see latero-). As an adjective from 1650s. Related: Quadrilaterally.