In Excel, a custom list is a predefined set of values that you can use to sort or fill data in a user-defined order. Excel comes with built-in lists for days of the week and months of the year, but you can also create your own. For example, you might have a list of department names, project stages, or priority levels that you frequently use. By creating a custom list, you can quickly autofill cells with this sequence or sort data according to it.