Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In geometry, a deltoid, also known as a tricuspoid, is a hypocycloid of three cusps. In other words, it is the roulette created by a point on the circumference of a circle as it rolls without slipping along the inside of a circle with three times its radius. It can also be defined as a similar roulette where the radius of the outer circle is 3/2 that of the rolling circle. It named after the Greek letter delta which it resembles.