As a noun, it is something that must happen or be done before something else can happen. As an adjective, it describes a required condition that comes first.
From Latin *prae* meaning 'before' and *requisitus* meaning 'asked for' or 'required'. The word came through French into English, keeping the sense of 'required beforehand'.
A prerequisite is literally something 'asked for before' you get access to the next step. School uses it for classes, but life uses it for skills—like speaking before debating. Once you see the 'pre' + 'requisite', schedules start to look like chains of hidden locks and keys.
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