An eighteen-sided shape is called an octakaidecagon, or octadecagon.
A shape with 7 sides is a heptagon
you call a two dimensional shape with six sides a hexagon
A shape with 8 sides is an octagon.
Triangle is a shape with three sides.
A regular shape.
It is an irregular shape
a shape with nine sides is called nonagon
A shape with straight sides is called a polygon. But it can have 3 or 4 sides as well.
octadecagon or octakaidecagon
If you say this shape is 18 sided i would guess it has 18 sides. ;)
A shape with 5 sides is a pentagon
A TRIGON , more usually a 'Triangle'.