Original: Once a bill has been formally approved by the executive ( in presidential systems ) or the formal head of government ( in parliamentary systems ) and consequently become law ( '' adopted '' ) , it is called an '' act '' , but the two terms are often used interchangeably in popular usage ( or some moments of scholarly imprecision ) .
Simplified: Once a bill has become law , it is thereafter an act ; but in popular usage the two terms are often treated as if they meant the same .