I wonder if there is a mathematical term or way to describe a continuous distribution with a "curved" boundary. By "curved" I mean the distribution has compact support whose boundary cannot be described by straight lines (planes, etc) or intervals.
Positive example: if we truncate a mixture of two two-dimensional Gaussians at density $p(x)\ge0.01$, its boundary shape may look like digit 8 (without holes), as illustrated below:
The boundary shape has no line segments.
Negative example: the boundary of two-dimensional uniform is square $[a,b]\times[c,d]$.