Illustrated Glossary of Organic Chemistry

Enantiotopic: A set of atoms or groups whose replacement leads to enantiomers.

The secondary hydrogen atoms of butane are enantiotopic because their replacement with a bromine atom forms either (R)-2-bromobutane or (S)-2-bromobutane, a pair of enantiomers.  However, the methyl hydrogen atoms of butane are homotopic (they are equivalent) because replacing any one of these six hydrogen atoms with a bromine atom leads to the same molecule, 1-bromobutane.