A frown was a facial expression that involved showing disapproval.
In 2376, while talking to Kes, Neelix said, "I'm not having a great day, either. Nobody seems to like today's menu. I tried blaming it on a replicator malfunction, but no one believed me. Wait, I, I think I see something. Neelix to Security, Kes' frown is losing its structural integrity. Red alert, I'm detecting a smile. That's my Kes." (VOY: "Fury")