In a chain of atoms A-B-C-D, the dihedral angle between the plane containing the atoms A,B,C and that containing B,C,D. In a Newman projection the torsion angle is the angle
(having an absolute value between 0° and 180°) between bonds to two specified (fiducial) groups, one from the atom nearer (proximal) to the observer and the other from the
further (distal) atom. The torsion angle between groups A and D is then considered to be positive if the bond A-B is rotated
in a clockwise direction through less than 180° in order that it may
eclipse the bond C-D: a negative torsion angle requires
rotation in the opposite sense. Stereochemical arrangements corresponding to torsion
angles between 0° and ±90° are called syn (*s*),
those corresponding to torsion angles between ±90°
and 180° anti (*a*). Similarly, arrangements corresponding to
torsion angles between 30° and 150° or between -30° and -150° are called clinal (*c*) and those between 0° and 30° or
150° and 180° are called periplanar (*p*).
The two types of terms can be combined so as to define four ranges of torsion angle; 0° to 30° synperiplanar (*sp*);
30° to 90° and -30° to -90° synclinal (*sc*);
90° to 150°, and -90° to -150° anticlinal (*ac*);
±150° to 180° antiperiplanar (*ap*).
The synperiplanar conformation is also known as the syn- or cis-conformation; antiperiplanar as anti or trans and synclinal as gauche or skew. For macromolecular usage the symbols
,
,
,
,
and
are recommended (*ap*, *sp*, *+sc*, *-sc*, *+ac*
and *-ac* respectively).

Source:

PAC, 1996, *68*, 2193*
(Basic terminology of stereochemistry (IUPAC Recommendations 1996))
* on page 2220