IPA: /dɪˈfɛkʃən/
KK: /dɪˈfɛkʃən/
A situation where someone leaves a group or organization to join another, often for political reasons.
His defection from the party surprised everyone.
Defection is formed from "de-" (meaning down or away) and "facere" (meaning to do or make). The word describes the act of withdrawing or abandoning a duty or allegiance, essentially doing away with one's previous commitments.
Think of 'doing away' ('de-') with a commitment or allegiance — that's what defection means.