In healthy patients, most fluids are confined in three spaces of the body: the intravascular space (in the blood vessels), the intracellular space (in the cells), or the interstitial spaces. The interstitial spaces are the small spaces between cells that are not contained within blood vessels. In normal situations, fluid moves between all three spaces to maintain fluid balance. Third-spacing occurs when too much fluid moves from the intravascular space into the interstitial or “third” space.

Reference:
Holcomb, S. (2009). Topics in Progressive Care: Third-Spacing: When Body Fluids Shifts. Nursing in Critical Care, 4(2). doi: 10.1097/01.NURSE.0000325345.22694.44