坚定性

Adjective 坚定 (jiān-dìng, firm) + nominalizing suffix 性 (-xìng) → 'the quality of being firm'. The same pattern produces 灵活性 (líng-huó-xìng, flexibility), 重要性 (zhòng-yào-xìng, importance), 可能性 (kě-néng-xìng, possibility) — useful template for talking about abstract qualities.

Formal / written. In speech, 坚定 itself usually does the job: 他很坚定 (tā hěn jiāndìng, 'he is very firm'). Reach for 坚定性 in essays, editorials, and policy talk.