Trigonometry is the branch of mathematics that deals with relationships between the sides and angles of triangles, especially right triangles. It has been studied since ancient times by civilizations like Egypt, Mesopotamia, and India. Key concepts in trigonometry include trigonometric functions like sine, cosine, and tangent that relate ratios of sides of a right triangle to an angle of the triangle. Trigonometry has many applications in fields like astronomy, navigation, engineering, and more.