something that makes you notice, remember, or think about something
a reminder of the dangers of drinking and driving
Occasional bursts of gunfire are a reminder that the rebels are still active.
constant/painful/vivid etc reminder
The damaged church was preserved as a stark reminder of the horrors of war.
The drop in stock prices serves as a reminder (=is a reminder) that investing is a form of gambling.
something, for example a letter, that reminds you to do something which you might have forgotten:
a reminder from the dentist for your check-up