The document discusses key terms and theorems related to circles, including: - Definitions of circles, radii, diameters, chords, secants, tangents - Types of circle intersections and relationships (concentric, tangent) - Theorems about tangents being perpendicular to radii and lines perpendicular to radii being tangents - Using properties of tangents, radii, and perpendicular lines to solve problems about circles - The Pythagorean theorem can be used with right triangles formed with radii and tangent segments