Window functions in SQL: why are they called window functions?

I understand window function but don’t understand why they are called window functions. When we say the “window” is it both the partition and the order by clauses?

Answer

Window functions operate on a range of rows, defined by the OVER clause. It is like you are looking through the window, you see sky, star, moon. You are not seeing the whole sky, you see part of the sky.

I came across this beautiful definition of window Function in simple-talk website. I am reproducing below for more clarity.

Original article on Simple-talk on window functions

Window (also, windowing or windowed) functions perform a calculation over a set of rows. I like to think of “looking through the window” at the rows that are being returned and having one last chance to perform a calculation. The window is defined by the OVER clause which determines if the rows are partitioned into smaller sets and if they are ordered. In fact, if you use a window function you will always use an OVER clause. The OVER clause is also part of the NEXT VALUE FOR syntax required for the sequence object, but, otherwise it’s used with window functions.

The OVER clause may contain a PARTITION BY option. This breaks the rows into smaller sets. You might think that this is the same as GROUP BY, but it’s not. When grouping, one row per unique group is returned. When using PARTITION BY, all of the detail rows are returned along with the calculations. If you have a window in your home that is divided into panes, each pane is a window. When thinking about window functions, the entire set of results is a partition, but when using PARTITION BY, each partition can also be considered a window. PARTITION BY is supported – and optional – for all windowing functions.

The OVER clause may also contain an ORDER BY option. This is independent of the ORDER BY clause of the query. Some of the functions require ORDER BY, and it’s not supported by the others. When the order of the rows is important when applying the calculation, the ORDER BY is required.

Window functions may be used only in the SELECT and ORDER BY clauses of a query. They are applied after any joining, filtering, or grouping.