origin:  Greek; enantíos= opposite + méros= part.
Two-faced: duplicating each side of the same human face as one.
An optical isomer; two sets of atoms that are mirror images of one another, but do not overlap if placed on top of one another (common example: your right & left hand, identical but opposites, a.k.a. chirality).
Now, just because something looks alike,and is made up of the exact same substances, it does not mean they are alike; so, in a curiosity of chemistry (even of the universe itself), what is harmless in one arrangement can become deadly when simply flipped.
You know...like "evil" Star Trek?
The drug thalidomide is an example of a molecule possessing an enantiomer that went horribly wrong (due to that "flipped" nature), it was designed simply to help women struggling with morning sickness & sleep depravation, which it did effectively, yet it was also capable of causing an array of serious birth defects. Removing the harmful enantiomer that causes birth defects (while leaving the useful one that causes sleep intact) is ineffective, for the molecule may re-flip inside the body.
The thalidomide tragedy forced drug companies to reconsider enantiomers as separate molecules instead of different forms of the same drug. However, it's still effectively used to treat leprosy and cancer with the enforced use of birth control.