Illustration of decagon
decagon
noun
deca·gon ˈde-kə-ˌgän 
: a plane polygon of 10 angles and 10 sides
Word History
Etymology
New Latin decagonum, from Greek dekagōnon, from deka- deca- + -gōnon -gon
First Known Use
circa 1639, in the meaning defined above
Kids Definition
decagon
noun
deca·gon ˈde-kə-ˌgän
: a closed figure having ten angles and ten sides