Define "chiral" in the context of organic molecules.

The definition of "chiral" in organic chemistry pertains to the spatial arrangement of atoms within a molecule. A chiral molecule is one that has non-superimposable mirror images. This means that when the molecule and its mirror image are compared, they cannot be aligned perfectly; they are distinct forms, much like a person's left and right hands.

This non-superimposability arises from the presence of a chiral center, most commonly a carbon atom that is bonded to four different substituents. Because of the specific arrangement of these groups around the chiral carbon, the molecule can exist in two different forms known as enantiomers, which possess different optical activity.

In contrast, the other choices provide definitions or characteristics that do not align with the true nature of chirality. Identical mirror images, linear structures without carbon chains, and molecules containing only one type of substituent do not capture the essential characteristic of chirality, which is the distinctiveness and non-superimposable nature of the mirror images. Thus, the correct understanding of "chiral" is embodied in the characteristic of having non-superimposable mirror images and typically includes a chiral center with four different substituents.