Interstitial fluid is a solution that bathes and surrounds the cells of multicellular animals. It is the main component of the extracellular fluid

Extracellular fluid

Extracellular fluid usually denotes all body fluid outside of cells. The remainder is called intracellular fluid.In some animals, including mammals, the extracellular fluid can be divided into two major subcompartments, interstitial fluid and blood plasma...

, which also includes plasma

Blood plasma

Blood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...

 and transcellular fluid

Transcellular fluid

Transcellular fluid is the portion of total body water contained within epithelial lined spaces. It is the smallest component of extracellular fluid, which also includes interstitial fluid and plasma. It is often not calculated as a fraction of the extracellular fluid, but it is about 2.5% of the...

. The interstitial fluid is found in the interstitial spaces, also known as the tissue spaces.

On average, a person has about 11 litres (2.4 imperial gallons or ~2.9 US gal) of interstitial fluid, providing the cells of the body with nutrients and a means of waste removal.
Plasma and interstitial fluid are very similar.