An electrolyte is broadly defined as any substance that passes electric current by the transport of ions. Electrolytes may be liquid solutions, fused salts or ionically conducting solids. An electrochemical process is a reaction that occurs across an interface, such as that between an electrolyte and an electrode. No electrochemical process can occur in isolation, that is a complete circuit must exist. Two electrodes separated by an electrolyte and connected by an external circuit (e.g., a wire) form a complete electrochemical cell. Thus, an electrolytic capacitor in circuit is an electrochemical cell.
Some compounds possess the ability to dissociate or ionize into positively and negatively charged species in solution. A common example of this is ordinary table salt, sodium chloride. Salts, however, are a subclass of all ionizing compounds because they exist as discrete ions in the solid state. Many molecular compounds, such as adipic acid dissociate in solution.
The anode oxide is formed on etched aluminum foil in an electrochemical process involving a so called formation electrolyte. Generally, these are aqueous solutions. Formation electrolytes are optimized to form aluminum oxide at maximum efficiency and highest oxide quality. Being aqueous, their use in finished capacitors is usually incompatible with the requirements of modern capacitors. The most obvious being the wide temperature range over which they are specified, e.g., -40°C to greater than 105°C. At lower temperatures aqueous systems solidify, resulting in loss of capacitance and excessive dissipation factors and ESR's. An aqueous electrolyte vaporizes in the upper temperature regime, accelerating the occurrence of the typical open circuit end-of-lifetime failure mode.
Thus modern capacitor electrolytes are based on such non-aqueous solvents as:
* ethylene glycol
* gamma-butyrolactone (GBL)
* dimethylformamide (DMF)
* N-methylpyrrolidinone (NMP)
* other solvents
* mixtures of the above
Usually, the concentration of water is maintained between 2 to 10 weight percent in order to balance physical and electrical properties. As operating temperatures of up to 150°C are specified, however, the trend towards completely "anhydrous" electrolytes has intensified. Nevertheless, the importance of water for proper oxide maintenance has been demonstrated. It is significant that paper contains approximately 8% w/w moisture at 73°F and 48% relative humidity because it is hygroscopic.
Electrolytes are characterized by their specific resistivity. For the sake of brevity, resistivity will be taken to mean specific resistivity. A more physically meaningful term is the equivalent conductivity, which considers the concentration number of ions generated by each compound in solution.

This definition is in context to Capacitor. See more contextual defintions for electrolyte, driving.