array formulas
formulas that operate on many values at once and return many results.
An ordinary formula takes single values and returns one result. An array formula takes ranges and works on every element, either returning one aggregate value or a whole block of results.
Modern spreadsheets evaluate array formulas automatically: =A1:A5*2 returns five results. Older ones required a special key combination, which is why array formulas are sometimes called CSE formulas.
When an array formula returns multiple values it needs empty cells to write them into. If those cells are blocked, the formula fails with #SPILL! rather than writing a partial answer.