A deltoid is the concave triangular curve formed when a small circle rolls around the inside of a circle three times as big. Eric Weisstein's Mathworld has a nice animation as well as a description of its properties.
I use deltoids for the ends of the cartouches in the sidebar and the icon that marks the end of each post.
So why did I call this thing "deltoid"? Well, it's named after the computer I use to write these postings. How geeky is that? I name all my computers after plane curves (I have another one called 'cycloid'). My wife insisted on regular names for our children, however. (And she vetoed 'Amber' as a name as well.)
If you've ever played with Spirograph you might have drawn this curve. Here's a spirograph applet (courtesy of Anu Garg) that lets you create this curve and other pretty ones: