Alternate history is a novel where some event in the past is different than our history. The cliche example is what would the late 20th century look like if the Confederacy had won the Civil War? The characters and story are set in this alternate time line and all come from that time line. This is not time travel, although cross-time stories are a way for a person from our time line to explore such a society. Time travel stories can generate an alternate history, but the key to alternate history is that the story is set some time after the change in history.