First Take: Used both in the general sense, as a journalist who engages in the business of uncovering a scandal or corruption,and to refer to a famous generation of writers and reporters at the turn of the twentieth century, some of whom were journalists (Lincoln Steffens, Ida Tarbell), and others who used social fiction in reform causes (Upton Sinclair, David Graham Phillips).

Deeper: Because the term originated in a speech in which Theodore Roosevelt attacked such writers, some associations of the term remain pejorative—e.g. associating this style of reporting with scandal-mongering and sensationalism. And indeed, classic muckraking did often draw upon conventions and vocabularies of nineteenth-century sensationalism and even melodrama.  But for modern reporters who typically engage in exposé, usually the term is gladly accepted as a badge of honor.

 Recommended Reading:  Cecilia Tichi, Exposés and Excess: Muckraking 1900/2000  (Philadelphia:  Univ. of Pennsylvania Press, 2004); Gretchen Soderlund, Sex Trafficking, Scandal, and the Transformation of Journalism, 1885-1917 (Chicago: University of Chicago Press, 2013).