Term used for different and indeed incompatible theories.
It has two main senses. First, that historical events must be seen in their uniqueness and can only be understood against the background of their context. In this sense it is akin to the emphasis on Verstehen in Wilhelm Dilthey's hermeneutics.
The second sense is that of Karl Raimund Popper (1902-1994) who uses 'historism' for the above sense. Historicism for Popper is the view that history is governed by inexorable laws, which the historian tries to predict, and is thus assimilated to science in a way quite incompatible with any appeal to Verstehen.
Popper's historicism (which he was concerned vigorously to oppose) also gives corporate wholes a life of their own which cannot be explained in terms of the individuals composing them.
Also see: holism
K R Popper, The Poverty of Historicism (1957)