placard

US /ˈplæk.ɑːrd/
UK /ˈplæk.ɑːrd/
1.

tấm áp phích

a large piece of card, paper, etc. with a message written or printed on it, often carried in public places by people who are complaining about something:

:
Demonstrators marched past holding/waving placards that said "Send food, not missiles!"