An ionic liquid is a liquid that contains only ions. In the broad sense, this term includes all the molten salts, for instance, sodium chloride at temperatures higher than 800 °C. Today, however, the term "ionic liquid" is used for the salts whose melting point is relatively low (below 100°C). In particular, the salts that melt at room temperature are called "room-temperature ionic liquids" (RTILs).

Room temperature ionic liquids consist of bulky and asymmetric organic cations such as 1-alkyl-3-methylimidazolium, 1-alkylpyridinium, N-methyl-N-alkylpyrrolidinium or ammonium ions. A wide range of anions is employed, from simple halides which inflect high melting points, to inorganic anions such as tetrafluoroborate and hexafluorophosphate and to large organic anions like bis-trifluorsulfonimide, triflate or tosylate. As one example, the melting point of 1-butyl-3-methylimidazolium tetrafluoroborate is about -80 °C, and it is a colorless liquid with high viscosity at room temperature.

The notable characteristics of ionic liquids are their non-measurable vapor pressure, non-flammability, thermal stability, wide liquid range, and solvating properties for diverse kinds of materials. It is known that, as in conventional organic solvents, many kinds of chemical reactions such as Diels-Alder reactions and Friedel-Crafts reactions occur in ionic liquids. Another important feature of ionic liquids is their designability: miscibility with water or organic solvents can be tuned through sidechain lengths on the cation and choice of anion. Furthermore, they can be functionalized to act as acids, bases or ligands.

Because of their properties, ionic liquids attract great attention in many fields, including organic chemistry, electrochemistry, physical chemistry, and engineering; see for instance magnetic ionic liquid. In addition, their non-volatility results in low impact on the environment and human health, and they are recognized as solvents for green chemistry.