Implicit bias is a form of bias against members of a group in virtue of their membership in that group. Implicit bias is distinguished from other forms of bias by the fact that it is unconscious. Because it is unconscious, it does not manifest itself in the form of explicit beliefs like "Women are not as good at philosophy as men," or "Minority students are not as logically rigorous as white students." Implicit bias may be based on these very stereotypes, but instead it manifests as an automatic judgment that, for example, a specific female does not seem talented in philosophy, or that a specific minority student will probably struggle on the next exam.
There is extensive evidence that implicit bias is significant and pervasive (see Further Discussion below). One study in which participants were asked to evaluate various Curricula Vitae showed that CVs with male names were evaluated far more favorably than identical CVs with female names. Evaluators were far more willing to hire male candidates, giving better marks for their teaching, research, and service (Steinpreis et al, 1999). This was true of both male and female evaluators.
Implicit bias can affect the classroom environment in many ways. For instance, a woman may make a point which is ignored, while the same point when made by a man is received and perhaps lauded. Additionally, minority students may not be recognized as being especially promising, since they do not fit the stereotype of what a "good philosopher" looks like. Implicit bias can also affect grading. A white male who performs badly on an exam may be shown lenience, given the instructor's estimation that he is good at philosophy, but had an atypically bad showing.
Foster participation by women and minorities, and prevent the trend of the same students dominating discussion.
Updated 7/17/2014, with many thanks to Dan Lowe, PhD candidate and member of the climate committee