nonagon
Concise Oxford English Dictionary © 2008 Oxford University Press:
nonagon /ˈnɒnəg(ə)n/
▶noun a plane figure with nine straight sides and angles.
– derivatives
nonagonal adjective.
nonagonal adjective.
– origin C17: formed irregularly from L. nonus ‘ninth’, on the pattern of words such as hexagon.
'nonagon' also found in these Oxford entries:

