A string identifying a particular cell in a spreadsheet, possibly relative to the cell containing the reference. A cell reference may be absolute (denoted by a "$" prefix in Excel) or relative (no prefix) in each dimension, thus, e.g. B$6 refers to the second cell across in the sixth row. The distinction between absolute and relative is only significant when the referring cell is copied, e.g. if cell A1, which refers to B$6, is copied to cell B1, then B1 will refer to C6.
If the reference is to a cell in a different sheet then it is prefixed with the target sheet's name and an exclamation mark. E.g. "Sheet 1!B3".