Here is a list of few things that you might call a "reminder":
Shopping list
Christmas present list
To-do list
Alarm clock
Egg timer
Birthday reminder
Now, is there a single way to describe all of those things? Well, it may get wordy, but a reminder could be described as:
A notification message or sound that either appears automatically, or shows when you look for it, which is used to let you know that a certain time has passed, a moment has arrived, or that an outstanding task has not been completed.
See, pretty wordy. Breaking it down like that helps to see what features a reminders app will need to have. Before getting to that, let's test the definition against the examples from here