"A historian" (also "a historic," "a historical," "a history," etc.). You don't normally use the indefinite article "an" except to preface a word that begins with a vowel sound (such as "an hour"). Some folks that don't pronounce the 'h' would use "an" ("An 'istoric moment."), and that is true of some historical colloquial pronunciations - which is why if you're reading a work from the 1700s you might run across "an horrific event" - which would have been read and pronounced "an 'orrific event" at the time it was written, but today looks weird because we actually pronounce the "H" - and because of the confusion, "an historic" tends to still be found and used today.

So for example, you have An Historical Essay on Architecture (1835), which is a period use of the term, and you have Making Sense of an Historic Landscape (2012) which...isn't.

I am not sure how much (if at all) this owes to shifting pronunciations, like the British tendency over the last couple centuries to pronounce the 'H' in words where they were formerly silent, like "herb" - so that it is now appropriate in British English to say and write "a herb" (a HER-b) rather than "an herb" (an erb). English tends to be idiosyncratic and complicated like that; it is a living language after all.