If a cell is placed in a solution of pure water, the tonicity of the solution is hypotonic.
A hypotonic solution has a lower concentration of solutes (e.g. salts, sugars) than the concentration of solutes inside the cell. In this case, pure water has no solutes dissolved in it, so it has a much lower concentration of solutes than the cytoplasm of the cell, which contains a variety of dissolved solutes such as salts, sugars, and proteins.
When a cell is placed in a hypotonic solution, water moves into the cell across its semi-permeable membrane, down its concentration gradient, from an area of high water concentration (the solution) to an area of lower water concentration (inside the cell). This can cause the cell to swell and potentially burst (lyse) due to the increased pressure inside the cell.
Therefore, if a cell is placed into a solution of pure water, the solution is hypotonic.