Efficiency of power converters
For those being confused like me, especially with the added comments, this is the relevant part taken from Wikipedia:


  Several ratios are used to quantify the function and performance of
  rectifiers or their output, including transformer utilization factor
  (TUF), conversion ratio (η), ripple factor, form factor, and peak
  factor. The two primary measures are DC voltage (or offset) and
  peak-peak ripple voltage, which are constituent components of the
  output.
  
  Conversion ratio (also called "rectification ratio", and confusingly,
  "efficiency") η is defined as the ratio of DC output power to the
  input power from the AC supply. Even with ideal rectifiers, the ratio
  is less than 100% because some of the output power is AC power rather
  than DC which manifests as ripple superimposed on the DC waveform. The
  ratio can be improved with the use of smoothing circuits which reduce
  the ripple and hence reduce the AC content of the output. Conversion
  ratio is reduced by losses in transformer windings and power
  dissipation in the rectifier element itself. This ratio is of little
  practical significance because a rectifier is almost always followed
  by a filter to increase DC voltage and reduce ripple. In some
  three-phase and multi-phase applications the conversion ratio is high
  enough that smoothing circuitry is unnecessary. In other circuits,
  like filament heater circuits in vacuum tube electronics where the
  load is almost entirely resistive, smoothing circuitry may be omitted
  because resistors dissipate both AC and DC power,so no power is lost.


(emphasis mine)

So it would probably be better to name it conversion ratio and not efficiency even though it is a ratio of powers.