30 entries under hard topic. ×

6
close(tough, solid, rigid, inflexible)
6
impenetrable(solid, rigid, resistant, impervious)
6
impermeable(adjective, solid, rigid, resistant)
5
compact(solid, rigid, density)
5
dense(solid, rigid, resistant, impervious)
5
firm(adjective, solid, rigid, inflexible)
4
concrete(tough, solid, rigid, inflexible)
4
thick(solid, rigid, syrupy, jellied)
3
solid(rigid, resistant, impervious, density)
3
substantial(solid, rigid, density)
2
compressed(tough, solid, rigid, inflexible)
2
concentrated(tough, solid, rigid, inflexible)
2
condensed(tough, solid, rigid, inflexible)
2
heavy(tough, solid, rigid, inflexible)
1
close-grained(tough, solid, rigid, inflexible)
1
congealed(tough, solid, rigid, inflexible)
1
fine-grained(tough, solid, rigid, inflexible)
1
hardened(tough, solid, rigid, inflexible)
1
impervious(tough, solid, rigid, inflexible)
1
inelastic(tough, solid, rigid, inflexible)
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APASmart Define. (2015). nonporous thesaurus. Retrieved September 29, 2026, from https://smartdefine.org/nonporous/thesaurus/synonyms
ChicagoSmart Define. 2015. "nonporous thesaurus" https://smartdefine.org/nonporous/thesaurus/synonyms (accessed September 29, 2026).
HarvardSmart Define 2015, nonporous thesaurus, Smart Define, viewed 29 September, 2026, <https://smartdefine.org/nonporous/thesaurus/synonyms>.
MLASmart Define. "nonporous thesaurus" 14 April 2015. Web. 29 September 2026. <https://smartdefine.org/nonporous/thesaurus/synonyms>