As a Noun: 1. SEROTONIN is a chemical produced naturally in your brain that affects the way you feel, for example making you feel happier, calmer, or less hungry. Note: 1. There appears to be no commonly used Auslan sign for SEROTONIN. If you know a sign for SEROTONIN used by you or other Auslan users (deaf people or interpreters) please go to "report missing sign" and supply details. Thank you.