From meta (change) and allassō (to change). To exchange one thing for another, to swap. Paul uses this word in Romans 1 to describe humanity's fatal exchange of truth for a lie.
In Romans 1:25-26, Paul describes the root of human depravity: people exchanged the truth about God for a lie and worshiped created things rather than the Creator. This exchange is the fundamental human sin — trading the infinite glory of God for finite substitutes. Every form of idolatry is an exchange: trading eternal satisfaction for temporary pleasure, divine truth for human opinion, the Creator for the creature.