Enantiomers are a type of stereoisomer that are non-superimposable mirror images of each other. These molecules have the same molecular formula and connectivity of atoms but differ in their three-dimensional arrangement. Enantiomers typically occur in chiral molecules, which possess a carbon atom bonded to four different substituents, leading to two distinct spatial configurations.
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Enantiomers are a pair of nonsuperposable mirror-image molecules. The D and L configurations of alanine are an example; glycine has no such pair because its amino acid alpha carbon carries two identical hydrogen chemical atoms. [ref]
Organic Chemistry by John McMurry. §26.1, discussion of glycine and Fischer projections after Table 26.1
The word names a relation between two stereoisomers. Merely finding two different spatial arrangements does not make them enantiomers: arrangements that are different but not mirror partners are diastereomers.
Key exmaple: d and L amino acids. Enantiomers have identical physico-chemical properties. But their biological roles can be very different, because an enzyme might only be able to act on one of them.