• for two straight lines, angle between the vectors associated with the lines, respectively;
• for a straight line and a plane, angle of the line and the orthogonal projection of the line upon the plane;
• for two planes, angle of two straight lines perpendicular to the two planes, respectively

NOTE 1 The SI unit of angle is the radian (symbol rad). Other units are the degree (symbol °), the minute (symbol ′) and the second (symbol ″): 1° = (π/180) rad, 1′ = (1/60)°, 1″ = (1/60)′.

NOTE 2 The term "plane angle" is only used in place of the term "angle" to distinguish it from the term "solid angle".