= Tautomer
A \b[tautomer] is one of the readily interconvertible <constitutional isomers> related by <tautomerism>. In a proton-transfer example, the forms retain the same atom inventory and net charge but differ in where a hydrogen is attached and in associated bond orders. A molecular formula alone therefore cannot identify which tautomer is meant.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry>[Entry T06252: tautomers and the prototropic case]{ref}
<Xanthine> supplies a concrete example. ChEBI distinguishes <7H-xanthine>, with the indicated hydrogen on nitrogen7, from <9H-xanthine>, with it on nitrogen9. Both neutral forms have formula $\mathrm{C_5H_4N_4O_2}$, and the curated records explicitly identify them as tautomers. The atom inventory is identical while the indicated hydrogen placement differs.<Xanthine in ChEBI>[CHEBI:17712 and CHEBI:48517, definitions, formula/charge and tautomer relations]{ref}
Interconvertibility does not mean equal abundance. For an invented dilute two-tautomer equilibrium $A\rightleftharpoons B$ with $[B]/[A]=0.01$, the fraction in form $B$ is $0.01/(1+0.01)=1/101$, about0.99%. This is a bookkeeping example, not a measured xanthine ratio. Solvent, temperature and interactions with other molecules can affect which form is favored.
A <tautomer> is not merely a <resonance (chemistry)> drawing: atom attachments change between tautomers. It is also not automatically a different protonation state, because gaining or losing a proton changes the atom inventory or charge rather than just relocating an existing hydrogen.
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