An Etymological Dictionary of Astronomy and Astrophysics

1. General: A → dimensionless number characterizing a porous medium, expressed by the ratio of the volume occupied by the pores to the total volume of the medium.
   

2. Geology: The percentage volume of hollow spaces in a rock, sediment, or soil.
   

3. Dust grains: The quantity usually defined as:

P = (V_v/V_t) = (1 - V_s/V_t), where V_v and V_s are the volumes of vacuum and of the solid matter making up the particle, and V_t is the total volume of the particle within some defined surface.

See also: → porous; → -ity.