Historicism is the eschatological position that says the thrust of prophecy in the Olivet Discourse and Revelation are fulfilled in the span of the church age. So for example, they would say that the great tribulation was not fulfilled in the first century, nor is it to be fulfilled in the future. Instead, it spans the entire church age (i.e. inter-advental).
You may by surprised to learn that most evangelical scholars are not preterists or futurists—they are historicists. But many are a mixture of preterist-historicist.
There are at least four reasons why Historicism is not a valid interpretation.
For then there will be great tribulation unlike anything that has happened from the beginning of the world until now, or ever will happen. And if those days had not been cut short, no one would be saved. (Matt 24:22)