decagon
Concise Oxford English Dictionary © 2008 Oxford University Press:
decagon /ˈdɛkəg(ə)n/
▶noun a plane figure with ten straight sides and angles.
– derivatives
decagonal adjective.
decagonal adjective.
– origin C17: via med. L. from Gk dekagōnon, neut. of dekagōnos ‘ten-angled’.
'decagon' also found in these Oxford entries:

