"Absolute Power Corrupts Absolutely" refers to who?

The famous saying, "Power tends to corrupt, and absolute power corrupts absolutely," is often called "Lord Acton's dictum". What was he referring to?

From Wikipedia:

In 1870 came the great crisis in Roman Catholicism over Pope Pius IX's promulgation of the dogma of papal infallibility. Lord Acton, who was in complete sympathy on this subject with Döllinger, went to Rome in order to throw all his influence against it, but the step he so much dreaded was not to be averted. The Old Catholic separation followed, but Acton did not personally join the seceders, and the authorities prudently refrained from forcing the hands of so competent and influential an English layman. It was in this context that, in a letter he wrote to scholar and ecclesiastic Mandell Creighton, dated April 1887, Acton made his most famous pronouncement:

"I cannot accept your canon that we are to judge Pope and King unlike other men with a favourable presumption that they did no wrong. If there is any presumption, it is the other way, against the holders of power, increasing as the power increases. Historic responsibility has to make up for the want of legal responsibility. Power tends to corrupt, and absolute power corrupts absolutely. Great men are almost always bad men, even when they exercise influence and not authority: still more when you superadd the tendency or certainty of corruption by full authority. There is no worse heresy than the fact that the office sanctifies the holder of it. "^[2]

Lord Acton is also credited with saying, “[History is] not a burden on the memory but an illumination of the soul.”

Indeed, history has proven his dictum, correct, and the dogma of papal infallibility, false. (See here, here, here, and here.)