What geometrical shape is a top hat?
If you are referring to an object of the form depicted by the link (at the end of the post),
it could be approximated by a 2-D figure which is then a concave polygon.
As a solid, it can be simulated by a cylinder sitting on a disk, or a truncated cone sitting on a disk, depending on the level of detail required.
http://www.google.ca/imgres?imgurl=http://www.rainbowengravers.com/CustomGraphics/tophat.gif&imgrefurl=http://www.rainbowengravers.com/CustomGraphics/index.html&usg=__7pPqwQIVbfzZlvL2NhQz1ZcUUyE=&h=598&w=692&sz=5&hl=en&start=10&zoom=1&tbnid=E9UkrTBYaL1unM:&tbnh=131&tbnw=152&prev=/images%3Fq%3Dtop%2Bhat%26um%3D1%26hl%3Den%26sa%3DN%26biw%3D967%26bih%3D463%26tbs%3Disch:10%2C349&um=1&itbs=1&iact=hc&vpx=436&vpy=163&dur=4369&hovh=209&hovw=242&tx=141&ty=161&ei=VHJ2TI2AJ8L6lweuhZXxCg&oei=N3J2TN-OLoW8lQfQr8zsCw&esq=12&page=2&ndsp=10&ved=1t:429,r:7,s:10&biw=967&bih=463