IPA: /ˌdɪsɪˈluːʒənmənt/
KK: /dɪsɪˈluːʒənmənt/
A feeling of disappointment that comes when someone realizes that something is not as good as they thought it was, especially when it involves a belief that is important to them.
After years of believing in the company's promises, she felt a deep sense of disillusionment when they failed to deliver.
Disillusionment is formed from "dis-" (meaning apart or away) and "illusion" (from Latin "illudere", meaning to mock or deceive). The suffix "-ment" indicates the process or result. Thus, disillusionment refers to the process of being freed from a false belief or deception.
Think of being 'freed from' ('dis-') a 'deception' ('illusion') — that's what disillusionment means.