Our management team ask us to join a pizza/beer bash, which is essentially nothing but a pizza lunch held for all employees.
My concept of the word bash still remained somewhere near the name of the famous Unix shell until I looked it up online. As expected, a bash means a party or a celebration. It's a slang.
Anyway, I have a hard time distinguishing between bash and party from online resources. Maybe someone who themselves has been to both of them could tell the subtle difference.