Retaliation occurs when an employer punishes an employee for engaging in legally protected activity. Retaliation can include any negative job action, such as demotion, discipline, firing, salary reduction, or job or shift reassignment. But retaliation can also be more subtle.
Sometimes it's clear that an employer's action is negative -- for instance, when an employee is fired. But sometimes it's not. In those cases, according to the U.S. Supreme Court, you must consider the circumstances of the situation. For example, a change in job shift may not be objectionable to a lot of employees, but it could be very detrimental to a parent with young children and a less flexible schedule.
As long as the employer's adverse action would deter a reasonable person in the situation from making a complaint, it constitutes illegal retaliation.