Reminder (noun)

1. A thing that causes someone to remember something.
2. A thing that prompts or reminds one of something.
3. A message, usually in the form of an email, that reminds the recipient of a task or event.

Origin:

Late middle english: from old french remin(d)eure, alteration of remen(d)eure, from re- "again" + men(d)eure "remainder".

Examples:

1. This photo is a reminder of our trip last summer.
2. I set a reminder on my phone to call my mother.
3. The smell of cookies was a reminder of her childhood.
4. He left a reminder note on the fridge.
5. The alarm clock is a reminder to wake up early.