Which electrical quantity is increased in a step-down transformer?

A step-down transformer decreases the alternating voltage from primary to secondary coils ($V_s<V_p$). Assuming an ideal transformer with negligible losses, power (wattage) remains constant ($P=V_p\cdot I_p=V_s\cdot I_s$). Because the voltage drops, the alternating current must increase proportionally ($I_s>I_p$) to keep the power level constant. Frequency remains unchanged.