What is Full scale deflection?
Yes.  Full scale deflection refers to the full range of motion of an analog 'needle' of an analog meter, or a galvanometer.  



In the above meter, the amount that the needle moves away from 0 is called the deflection.  Full scale deflection is, well, deflection that goes all the way to the end of the scale, in other words, full scale.  Anything beyond this scale is not really measurable by this insturment, and so the term 'full scale' actually originates from the older term and concept of 'full scale deflection'. Be it an analog coil-actuated meter, a digital ADC, or even a DAC (though it is used to mean the output range available, not measurement), full scale is now used to mean the complete minimum to maximum range of something. We've mostly dropped the deflection part however, as deflection is rarely relevant anymore.

Note that it need not be referring to voltage specifically. One could correctly say '0 to 100mA full scale' for example.