the.com/porosity

means The percentage of a material's volume that is actually empty space — tiny voids and pores tucked inside what looks like solid stuff.

from From 'porous,' tracing back to Greek 'poros' meaning a passage, channel, or ford — originally a way through, not a gap within. Latin picked it up as 'porus,' and by the time it settled into English scientific vocabulary, the word had turned inward: no longer a path you walk through, but the countless tiny paths hidden inside stone, bone, bread, and skin.