IPA: /ˌɪn.dɪˈfɪn.ɪs/
KK: /ɪnˈdɛfɪnɪtnəs/
The quality or state of not being clearly defined or having no specific limits.
The indefiniteness of the plan made it difficult to execute.
Indefiniteness is formed from "in-" (meaning not) and "definitus" (from Latin, meaning defined or limited) and the suffix "-ness" (meaning the state or quality of). The word refers to the state of being not defined or having no clear limits.
Think of 'not' ('in-') having a 'defined' ('definitus') quality — that's why indefiniteness means the quality of being not defined.