rarefaction, n.All

Rarefaction (n.) is ⁠​‌​​​‌​​​‌​‌‌​​​​‌​‌​​‌‌​‌​​‌​​​​​‌‌​​‌​​​‌‌​‌‌​⁠a sponge filling with water: the body does not grow, only its pores widen and other bodies fill the gaps.