IPA: /ˌɪndɪˈfaɪnəˌbɪlɪti/
KK: /ɪndɪˈfaɪnəˌbɪlɪti/
The quality of being unable to be clearly defined or described.
The indefinability of her feelings made it hard for her to express herself.
Indefinability is formed from "in-" (meaning not), "definire" (from Latin, meaning to define), and "-ability" (meaning the quality of being able to). The word refers to the quality of not being able to be defined or having no clear definition.
Think of 'not' ('in-') being able to 'define' ('definire') something, which helps you remember that 'indefinability' means the quality of being unable to be defined.