An unconscious bias is when discrimination and incorrect judgements occur due to stereotyping. These can occur automatically and without the person being aware of it. These types of biases are often so ingrained in culture and society they they go unnoticed by many people. This is most common in gender, age, and race stereotyping.
An example of unconscious bias could be seen in a job application and interview process. Some words that are used when describing a person can be perceived differently in regards to if it is describing a male or female candidate. The words bold, gutsy, and assertive are viewed as positive qualities for a male applicant while these same words are perceived as negative qualities for a female candidate.