An electrolyte is a liquid substance which acts as a medium to conduct electricity. One speaks of an electrolyte in a number of different contexts, the most common including health and fitness, and various electronics and automotive disciplines. An electrolyte is full of ions, which are atoms that have some sort of net electric charge, either positive or negative. A dilute electrolyte has a relatively small amount of ions for its volume, while a concentrated electrolyte has a high amount of ions.

In basic fuel cells, such as those used in automotive technology, an electrolyte acts as the liquid that allows ions to travel between the cathode and anode to keep the power-generating process underway, while keeping the reactive oxygen and hydrogen apart. In the type of fuel cell known as a proton exchange membrane cell, the electrolyte specifically moves protons (positively-charged hydrogen ions) to the cathode from the anode where they are produced, with the end result being the production of water and electricity.

Batteries also employ an electrolyte of some sort, both to conduct electricity between the battery plates, and to store energy on the plates themselves. The electrolyte used in a battery depends on the battery's type and purpose. Most car batteries, for example, use an electrolyte which contains sulfuric acid, which is why they require careful handling. Alkaline batteries use an alkaline solution for their electrolyte. Lithium batteries use a special organic electrolyte which freezes at much lower temperatures than more traditional water-based electrolytes.

In human health, electrolytes aid in a number of vital bodily processes. Many heart and nerve functions, muscle control and coordination, and the body's ability to absorb fluids all depend on a healthy balance of electrolytes. The most common electrolytes found in the human body are sodium, potassium, magnesium, chloride and calcium. Various hormones in the body help regulate intake of these electrolytes, and the kidneys filter electrolytes which reach excessive levels. Unhealthy levels of consumption of any or all of the body's necessary electrolytes can cause serious health issues. The most common imbalances are hyper- and hypo-natremia, or excessive and insufficient levels of sodium, and hyper- and hypo-kalemia, or excessive and insufficient levels of potassium.

Many sports drinks contain added potassium and sodium to help restore the body's proper electrolyte balance after intense physical exertion. After exercising heavily, it can be dangerous to ingest large quantities of water, as it may dilute existing electrolyte levels and cause physical problems. Additionally, a number of electrolyte enhanced drinks exist specifically for children and may be helpful for regular use, particularly to restore electrolyte balance after sickness which has included vomiting or diarrhea.