When people sing together for fun, especially with songs that everyone knows.
Singalongs are usually happy and enjoyable, and they often happen in social gatherings or at parties.
Examples:
We had a singalong at the campfire.
The children love singalongs at school.
She organized a singalong for her birthday party.
The whole family joined in the singalong.
The singalong was so much fun, everyone was singing and laughing together.
I'll never forget the singalong we had at the beach.