First take: Roughly synonymous with “journalist,” a term originally associated with the writer of a letter (or, in some cases, the person to whom one communicated in a letter). Today, the label is used primarily to identify a journalist in a foreign location, as in “foreign correspondent” or (more specifically) “war correspondent.” 

Deeper: The older sense of the term referred to the writer of longer feature articles sent back to domestic audiences, often within the more leisurely, reflective, and upper-class style of travel writing—a convention disrupted in war writing, of course, by the rapid-fire pulses of the telegraph. Into the mid-nineteenth century, nevertheless, the correspondent was not always a staff writer, nor even a professional journalist—it could be a foreign national, or a soldier.  Even today, the term can be used for a more occasional contributor, e.g. a journalist from another, affiliated news organization. That being said, the term has often been associated not just with the description of social conditions abroad, but with a political and/or economic assessment.

Recommended Reading:  Christopher P. Wilson, “Plotting the Border: Pancho Villa, John Reed and Insurgent Mexico,” in Amy Kaplan and Donald Pease, The Cultures of United States Imperialism: New Essays (Durham, N.C.: Duke University Press, 1993): 340-61.