Race is a social construct (i.e. man-made invention) based on skin colour and facial features (shape of eyes, nose, lips, hair texture, etc.). Even if it isn’t biologically “real”, its impact on people is very much a reality. Race is a system of social categorisation which advantages certain groups of people and disadvantages other groups of people.
Possible definition of race: fluid and erroneous system of classifying human beings that was invented by Europeans, and has consistently functioned over time to concentrate power with white people and legitimize dominance over people of colour. Race was invented for the purpose of economic and political gain in the context of colonisation and slavery and it continues to serve the purpose of legitimising white supremacy.
In Europe, race is also associated to religion: Nazis believed Jewish people were an “inferior race” and, more recently, discrimination against Muslim people or people who “look” Muslim tends to be classified as racism since it is linked to skin colour and facial features. Discrimination based on ethnicity is equally thought of as racism in Europe, although it should technically be called xenophobia rather than racism.