Definition of decagon noun from the Oxford Advanced Learner's Dictionary

 

decagon

 noun
noun
BrE BrE//ˈdekəɡən//
 
; NAmE NAmE//ˈdekəɡɑːn//
 
(geometry)
 
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a flat shape with ten straight sides and ten angles Word Origin mid 17th cent.: via medieval Latin from Greek dekagōnon, neuter (used as a noun) of dekagōnos ‘ten-angled’.
See the Oxford Advanced American Dictionary entry: decagon