hypotonic

A solution with less osmotic pressure or tonicity than plasma or interstitial fluid. This results in a net flow of water across semipermeable membranes. Thus bathing cells in culture within a hypotonic solution will cause water to diffuse into the cells across the cell membrane, causing the cells to expand or burst.

References

Dorland's Medical Dictionary, 23rd ed., WB Saunders, Philadelphia, 1990