Isomers have the same molecular formula but different arrangements of their atoms. Constitutional isomers differ in connectivity; stereoisomers retain connectivity but differ in spatial arrangement.
For example, ethanol and dimethyl ether share but connect their atoms differently. Two enantiomers instead have mirror-related spatial structures. Formula, connectivity, and stereochemistry therefore encode different levels of chemical identity.
A chemical substance with the same molecular formula as another but a different connectivity of chemical atoms.
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Isomers were quite confusing for early chemists, before atomic theory was widely accepted, and people where thinking mostly in terms of proportions of equations, related: Section "Isomers suggest that atoms exist".