Microaggression is a term coined in 1970 by Professor Chester Pierce of Howard University. Microaggression is defined as "brief and commonplace, daily verbal, behavioral, or environmental indignities, whether intentional or unintentional, that communicate hostile, derogatory, or negative racial slights and insults toward people of color."
Microaggressions: subtle, often automatic and subconscious verbal and non-verbal exchanges toward members of minority groups based on race, ethnicity, gender, sexuality, mental illness, etc.
Examples: dismissive looks, gestures, and tones, name calling, racial epithets, avoidance, rudeness, excluding, demeaning attitudes or thoughts, etc.
The University of Illinois started an interdisciplinary Microaggression research group and in 2015 published a report of their findings on students' experiences of microaggressions in the classroom. More information on the group and project can be found at their website: http://www.racialmicroaggressions.illinois.edu/