IPA: //ʌnˈsteɪbəl.nəs//
KK: /ʌnˈsteɪbəl.nəs/
The quality or state of being unstable, which means not firm or steady and likely to change suddenly.
The unstableness of the ground made it difficult to build a house there.
Unstableness → It is formed from "un-" (meaning not) and "stable" (from Latin "stabilis", meaning firm or steady) and the suffix "-ness" (meaning the state of being). The word "unstableness" refers to the state of not being firm or steady.
Think of 'not' ('un-') being combined with 'firm' ('stable') to remember that 'unstableness' means the state of being not firm.