ПЛОТНОСТЬ _|_ DENSITY / PLOTNOST [ˈpɫotnəsʲtʲ]
Hint with an alternative definition: it is the main feature of a pressured non-porous block with a load (mass physical object) which can float in water or an object with plenty of plates put one on one, strongly plaited & tied up.
Hint with composition: bulk of tons + a lot = density (all inner mass of an object, the more density, the more weight)
Hints with translation + consonant words (by sounds or letters): stout-built & plod person.
Mental visualization: ПЛ (two plates are connected to each other, seal) + О (completeness, no seal is required) + ТН (seal, like weaving threads)