Is it “an historic” or “a historic”?

A and an are different forms of the indefinite article. Words where the “h” sound is pronounced, such as “hat,” “hotel,” or “hard,” use “a” instead of “an.”

However, authorities disagree on whether “historical” should be preceded by “a” or “an.” Many dictionaries encourage you to use the indefinite article that suits your pronunciation (depending on whether you pronounce the “h” sound at the start of the word).