What is the definition of bias? In U. S History?

Bias means a strong inclination of the mind or a preconceived opinion about something or someone. A bias may be favorable or unfavorable: bias in favor of or against an idea.

-www.dictionary.com

Basically, bias means having an unfair or unbalanced opinion. Since history is a subject where people express their opinions it means that we have to be very careful to watch out for bias.

-community.dur.ac.uk

Explanation:

US History taught in schools is most of the time very biased, in an attempt to build patriotism early  in a young individual's mind. They rarely disclose the negative impact the US has had on many other countries (and if they do, it's often sugar-coated), ultimately focusing on the problems with other nations. They build a false superiority to other countries.

it's not the readmission