Frequency response of DC amplifier
Some amplifiers are incapable of amplifying low-to-zero frequencies.  Those are called 'AC amplifiers'.   Amplifiers which are capable of amplification at
low-to-zero frequency are 'DC amplifiers'.   There is, technically,
no use for absolute DC amplification (it would take a day of testing to
find out that an amplifier 'only' goes down to 0.000 002 Hz),
so the terms are applied loosely. 

When frequency response is plotted, measured points are at positive
frequencies.