Here is the list of a few of the things that you might call a "reminder":
A shopping list
A Christmas present list
A to-do list
An alarm clock
An egg timer
A birthday reminder
Now, is there a single way to describe all of these 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. It 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 this helps us to see what features a reminder app will need to have. Before getting to that, let's test the definition against the preceding examples:
A shopping list: In this case, you go looking for the reminder. Although, we could set it up to automatically show when your location happens to be near the store! Other than that, this is effectively a task that has not been completed.
A Christmas present list...