Definition from Wikipedia
“Dunbar's number is a suggested cognitive limit to the number of people with whom one can maintain stable social relationships—relationships in which an individual knows who each person is and how each person relates to every other person. By using the average human brain size and extrapolating from the results of primates, he proposed that humans can comfortably maintain only 150 stable relationships.”
Going further
I've curated 400+ definitions just like this one in a tiny dictionary. In this glossary you can easily access, search and understand 400+ terms that service designers and other innovators use.