A dihedral angle or torsional angle (symbol: θ) is the angle between two bonds originating from different atoms in a Newman projection.
eg: staggered conformation of ethane
The angle between any blue C-H bond (C-H1, C-H2, C-H3) and any red C-H bond (C-H4, C-H5, C-H6) is a dihedral angle. Thus, the angle between C-H1 and C-H4, which is 60º, is a dihedral angle.
Geometrically, a dihedral angle is an angle between two intersecting planes. For example, the dihedral angle between C-H1 and C-H4 in the above Newman projection is the angle between the plane bisecting C-H1 and that bisecting C-H4, both perpendicular to the plane of the screen.
see also bond angle