Wikipedia
Historism is a philosophical and historiographical theory, founded in 19th-century Germany (as Historismus) and especially influential in 19th- and 20th-century Europe. It pronounces the historicity of humanity and its binding to tradition.
Historist historiography rejects historical teleology and bases its explanations of historical phenomena on sympathy and understanding (see Hermeneutics) for the events, acting persons, and historical periods. The historist approach takes to its extreme limits the common observation that human institutions (language, Art, religion, law, State) are subject to perpetual change.
Historism is not to be confused with historicism, nevertheless the English habits of using both words are very similar. (The term historism is sometimes reserved to identify the specific current called Historismus in the tradition of German philosophy and historiography.)