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An ionic liquid is a liquid composed of ions, conventionally a low-melting salt. Usage often restricts the term to salts with melting or glass-transition temperatures below $100{}^{\circ }\mathrm{C}$; a room-temperature ionic liquid must additionally be liquid at the stated room temperature. IUPAC notes the conventional restriction rather than making $100{}^{\circ }\mathrm{C}$ a physical discontinuity.^[ref]

A neat ionic liquid differs from a salt dissolved in water. The latter contains a molecular solvent as well as ions; a dry pure ionic liquid does not require that solvent to remain liquid. Its cation and anion need not be small monatomic species. Molecular shape, charge distribution and interactions affect the solid–liquid free-energy balance; the presence of electric charge alone does not determine melting temperature.

For an original number-density example, stipulate a pure 1:1 ionic liquid with molar mass $200\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ and density $1.20\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$. One litre contains $1200\mathrm{g}$, hence $6.00$ moles of formula units, or $6.00$ moles of each ionic species. The combined stoichiometric ion count is therefore $12.0$ moles per litre. That is a composition calculation, not an ideal-solution activity or a prediction of conductivity: ionic correlations and mobility matter.

Przemyslaw Kubisa's polymerization review gives a motivating use. An ionic medium can dissolve reactive components and affect polymerization kinetics or facilitate separation of polymer from a catalyst-containing phase.^[ref] “Ionic” does not by itself guarantee low viscosity, chemical inertness, environmental harmlessness or compatibility with every reaction. The selected ions, water content, temperature and intended measurement must be specified.

Przemyslaw Kubisa studied the use of ionic liquids in polymerization. His 2004 review examined their solvent role, including effects on polymerization kinetics and separation of polymer from catalyst-containing reaction media.^[ref]

The solvent is part of the chemical system rather than merely an inert container. It can alter the environment of an active chain end, the solubility of growing chains and the partition of catalyst and product. Consequently equal monomer conversion in two media does not guarantee equal chain lengths or equal product purity.

An original ideal-chain calculation makes one distinction precise. Suppose $0.100\mathrm{mol}$ of monomer is combined with $0.001\mathrm{mol}$ of initiator, every initiator forms exactly one growing chain, and there is no chain transfer or termination. At conversion $x=0.80$, the degree of polymerization contributed by incorporated monomer isIf only half the initiator forms chains under the same assumptions, that average becomes 160, despite equal monomer conversion. Conversely, chain transfer can increase the chain count and lower the average.

The model is deliberately idealized, not a measurement in Kubisa's review. It shows why solvent comparisons need chain-count and molecular-size information as well as reaction yield. A solvent enabling convenient catalyst recovery still requires separate evidence for its influence on the polymerization mechanism.