noun Trigonometry. 1.a law stating that the square of a side of a plane triangle is equal to the sum of the squares of the other two sides minus twice the product of the other sides multiplied by the cosine of the angle between them.
2.a law stating that the cosine of an arc of a spherical triangle equals the sum of the product of the cosines of the other two arcs added to the product of the sines of the other two arcs multiplied by the cosine of the angle between them.