Historical determinism is the stance in explaining history or advocating a political position that events are historically predetermined (and/or currently constrained) by various forces. Since such explanation is the norm, it may be better understood in contrast to its negation, i.e. the rejection of historical determinism.
The italicized alternation (and/or) is significant because some political philosophies (e.g. Marxism) assert a form of the one and reject the other. Used as a pejorative, it is normally meant to designate an overdetermination of present possibilities by historical conditions.