Final answer:
In the INDEX function, the 'row number' refers to the number of the row in the range referenced in the first argument.
Step-by-step explanation:
In an INDEX function, the argument referred to in your question is the row number. Specifically, this argument determines which row in the selected array or range will be returned by the function.
The syntax of the INDEX function is typically INDEX(array, row_num, [column_num]), where array is the range of cells or table, row_num is the number of the row in the array from which to retrieve a value, and [column_num] is an optional argument that specifies the column from which to retrieve a value. If only the row number is specified, the INDEX function will return the entire row of that array.
Hence, in the INDEX function, the term 'row number' denotes the position of the row within the range specified in the first argument.