The intracellular fluid pertains to the internal fluid located within the cell(s). In humans, the total body water composition is made up mainly of intracellular fluid (67%) and extracellular fluid (26%). The intracellular fluid, in turn, is composed of water dissolved ions, and other molecules.
The intracellular fluid pertains to the cytosol(s) of the cell. The cytosol is the part of a cell where the cellular organelles are suspended. It contains mainly of water and a variety of substances, such as dissolved ions (e.g. sodium ions, potassium ions, etc.), macromolecules (e.g. proteins, nucleic acids), other metabolites, etc.
As opposed to the extracellular fluid, which is the bodily fluid outside the cells, the intracellular fluid contains more charged macromolecules. As for the ions, the cytosol has more potassium ions while has fewer sodium ions compared with the extracellular fluid. This is particularly important in osmoregulation.
The cytosol is also involved in cell signaling, e.g. calcium signaling. For instance, a hormone activates a cell to open its calcium channels to allow the entry of calcium ions. The influx of calcium ions leads to a cascade of reactions, such as the activation of other signaling molecules, calmodulin and protein kinase C.1
Word origin: Latin intra- ("inside of") + cellulāris, cellul(a), ("live cell")
1 Kikkawa, U., Kishimoto, A., & Nishizuka, Y (1989). "The protein kinase C family: heterogeneity and its implications". Annu. Rev. Biochem. 58: 31–44.