A tangent circle is a circle that touches another circle at exactly one point. This point of contact is called the point of tangency. Tangent circles can be either externally tangent, where they touch from the outside, or internally tangent, where one circle is inside the other and they touch at one point. In geometry, tangent circles are important for understanding relationships between shapes. They are often studied in relation to Euclidean geometry and can be used in various applications, including engineering and design. The concept helps in visualizing how circles interact with each other in a plane.