The term "spill range" refers to the range of values returned by an array formula that spills results onto a worksheet. This is part of Dynamic Array functionality in the latest version of Excel.
In the example shown, the formula in D5 is:
This formula returns seven unique values, and these results spill into the range D5:D11 automatically. The range D5:D11 is called the "spill range". When you select any cell in a spill range, entire range will be highlighted with a blue border.
Cell reference syntax
You can refer to a spill range with the first cell in the range and a hash character (#) like this:
=D5# // entire spill range
To count values returned to the spill range, you can write:
=COUNTA(D5#) // returns 7
To retrieve the 3rd value, you could use INDEX like this:
=INDEX(D5#,3) // returns "green"
If something on the worksheet blocks a spilled formula, it will return a #SPILL! error.