"Molecules which closely resemble each other, but fail to be identical due to one difference in their structures can be designated as isomers. When isomers have same atom connectivity but differ in spatial orientation of their atoms (configuration), they are designated as stereoisomers. The phenomenon of existence of stereoisomers is called stereoisomerism" ...

Read more @ the following article, link given below.

K. Valliappan, Isomerism and isomeric drugs, Indian drugs, 33(3), 98-106, 1996 and references therein.

https://chiralpedia.com/pdf/Indian-Drugs-Isomerism-and-Isomeric-Drugs.pdf