What does bias mean in history?

Bias in this context refers to the absence of objectivity. In historical practice, bias exists on two levels. The first is our own bias as historians. Whenever we try to understand historical events, we bring our own assumptions and beliefs, and these are shaped by our own times. Sometimes called "presentism" by historians, it is not good history, because it can tend to hold historical actors accountable to the standards of our time, as opposed to their own. We can see another level of bias when we read primary sources, which are the lifeblood of history. On the one hand, we depend on first-hand accounts and sources to make sense of the past. On the other hand, we have to remember that these accounts were generated by human beings, and that these humans brought their own biases to bear on the subject. For example, if we want to learn more about Native Americans, the writings and first-hand accounts of Europeans who came in contact with them are essential. But they are very problematic sources, because Europeans made assumptions about their own superiority, and in any case didn't really understand the culture of the people they were describing. Biases, both on the part of the historian and within the sources themselves, are unavoidable, and historians have to take them into account to do intellectually rigorous work.