In the literal sense, the bubble point is the point in time (point) when the first air bubble (bubble) appears on the other side after air pressure is applied to a hydrophilic membrane in water.
It is also known as bubble pressure and indicates the minimum pressure with which the capillary water is pushed out of the membrane pores. Capillary forces and surface tension keep the water in the pores. These retention forces are all the lower the larger the pores are. Therefore, the bubble point is the gauge of the largest pore diameter, because the air is first blown through this pore.
With hydrophobic membranes, a liquid other than water is used to wet these membranes, e.g. alcohol.