Inclination, leaning, prejudice, predisposition
A biased source is one in which the creator has a view of the issue at hand that had an effect on how they created the source. From the synonyms above, you can see that this can be to a small or large degree. Everyone has biases, and someone with a bias can still write a worthwhile source, but it is up to you to consider how much of a bias is present. Be aware of the biases inherent when an organization has a legislative agenda or is trying to sell something or is perhaps trying to appeal an audience with a set of investments and values.