The definition was actually for 4 °C, the point at which water is most dense. At 20 °C the density of water is about 0.997 g/mL. However, we don’t use water to define the metric system anymore, so even at 4 °C - or more precisely 3.983035(670) °C - water is not exactly 1 g/mL.
1 mL of pure water weighs exactly 1 g at 20 °C and 1 atm pressure :) It’s a defined standard, useful for calibrating other things.
The definition was actually for 4 °C, the point at which water is most dense. At 20 °C the density of water is about 0.997 g/mL. However, we don’t use water to define the metric system anymore, so even at 4 °C - or more precisely 3.983035(670) °C - water is not exactly 1 g/mL.