Explain the meaning of the statement: "Tonicity is only useful as a comparative term."

Tonicity is a comparative term that has no units. It is frequently used when describing what happens in the cell volume at equilibrium. Tonicity always compares a solution and a cell, and it is used to describe only a solution. Tonicity is related to its osmolarity. There are three terms, hypotonic, isotonic, and hypertonic that used to compare the extracellular fluid around.