A mermaid dress is a dress which is tight around the bottom and thighs, but which expands out around the lower legs, coming down to the ankles. The name comes from its resemblance to a mermaid's tail. In extreme instances, the dress may be so tight around the thighs that it acts rather like a hobble skirt.
Normally, such a dress would be made of satin. However, latex ones are available from some fetish suppliers.