IUPAC Atropisomers is defined as following
A subclass of conformers which can be isolated as separate chemical species and which arise from restricted rotation about a single bond, e.g. ortho-substituted biphenyl, 1,1,2,2-tetra-tert-butylethane.
The word atropisomer is derived from the Greek a, meaning not, and tropos, meaning turn. The name was coined by Kuhn in 1933, but atropisomerism was first detected in 6,6’-dinitro-2,2’-diphenic acid by Cristie in 1922.