Tangent circles are two circles that intersect only at a single point.

Tangent circles that have no common interior points are known as externally tangent. If one of the circles is located inside the other, they are said to be internally tangent.

Two or more tangent circles that are externally tangent may be the same size or different sizes. Internally tangent circles must always be of different sizes.

Two circles with radii $r_1$ and $r_2$ are externally tangent if the distance between their centers equals $r_1+r_2.$