Tangent circles are two circles that touch at exactly one point. This point is called the point of tangency. When two circles are tangent, they share a common tangent line at this point, which means the line touches both circles without crossing them. There are two types of tangent circles: externally tangent and internally tangent. In externally tangent circles, the circles touch from the outside, while in internally tangent circles, one circle is inside the other, touching at the point of tangency. Tangent circles are often studied in geometry and can be used in various applications, including engineering and design.