/ rɪˈmaɪndə(r); NAmE / noun
1.
reminder (of sb/sth) | reminder (that ... ) something that makes you think about or remember sb/sth, that you have forgotten or would like to forget :
The sheer size of the cathedral is a constant reminder of the power of religion.
The incident served as a timely reminder of just how dangerous mountaineering can be.
2.
a letter or note informing sb that they have not done sth