Isomer
Definition
Molecules with the same formula but different arrangements: same atoms, different structure, often very different properties.
Technical definition
Isomers are compounds with identical molecular formulas but different atom connectivity (structural isomers) or spatial arrangement (stereoisomers). They are distinct substances with different physical and often chemical properties.
Example
Butane and methylpropane are both C4H10, but butane boils at -0.5 C while methylpropane boils at -11.7 C: same atoms, different shape.
Common confusion
Isomers are not resonance structures. Isomers are separable, distinct compounds, while resonance forms are just different drawings of the same molecule's electron distribution.