IPA: /ˌæs.ɜːrˈteɪn.mənt/
KK: /æˈsɜrˌteɪn.mənt/
The process of finding out or discovering something with certainty.
The ascertainment of the facts was crucial for the investigation.
Ascertainment is formed from "ascertain" (from Old French *acertener*, meaning to make certain) and the suffix "-ment" (meaning the action or result of). The word refers to the action or process of making something certain or known.
Think of the action of making something 'certain' — that's what ascertainment means, and the '-ment' shows it's the result of that action.