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.
A racemate contains the same amount of substance of each member of an enantiomer pair. Their equal and opposite contributions cancel in ordinary solution polarimetry under identical conditions. Zero net optical rotation therefore does not establish achirality of the individual molecules.[ref]
5.8 Racemic Mixtures and the Resolution of Enantiomers. McMurry, Organic Chemistry, §5.8: equal enantiomer populations and cancelling optical rotations
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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.
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.