I am very guilty of using the term sensitivity and efficiency interchangeably and really they are, a more efficient speaker will play louder than a less efficient speaker, but in the truest sense...
Loudspeaker efficiency is defined as the sound flux or sound power at output divided by the electrical power input...
Sensitivity is how loud a speaker plays with a given input. I defer back to 1 watt of power measured 1 meter away anechoic.
Tim