It is called a hendecagon. To figure it out they follow a standard ending of "agon" which basically means shape or side.Squares and triangle also follow this convention just have more common names. The first part "hendec" refers to the number of sides. For instance pent means 5, so a pentagon is a 5 sided figure. A 16 sided figure also matches this. It's name his hexadecagon. Which hex means six, deca means 10 and gon means figure or shape. so 10+6=16 sides

...Hendecagon = 11 sided figure.

This is the order from 1 to 20:

Henagon=1

Digon=2

Triangle=3

Quadrilateral=4

Pentagon=5

Hexagon=6

Heptagon=7

Octagon=8

Enneagon=9

Decagon=10

Hendecagon=11

Dodecagon=12

Tridecagon=13

Tetradecagon=14

Pentadecagon=15

Hexadecagon=16

Heptadecagon=17

Octadecagon=18

Enneadecagon=19

Icosagon=20

There are two names that can be used to refer to an 11 sided figure. It can be called undecagon or hendecagon.

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