Another classical architectural term here. On a Doric capital, the echinus is circular block that supports the abacus, and its profile is a type of non-circular curve — i.e., it looks kind of “squished” (that’s a highly technical term, you know). The echinus can be plain and smooth or have egg-and-dart ornamentation. So why the heck is it called an echinus” Well, echinus is also the genus of sea urchins, and when you look at the profile shape of this type of molding, it kind of looks like the profile of a sea urchin — before it lands on your plate at a sushi bar, that is.
See also: abacus and egg-and-dart