Without getting into the abstract definition, algebra is basically just arithmetic ("regular math") using variables (like "x") to represent unknown values. Typically you're trying to find a value (a regular number) or an expression (a relationship between values and variables) equal to your variable.

When you "solve for x", you're doing algebra. If your Excel formulas reference cells as variables (e.g. "=A1*3") then yes that is algebra, since the cell can take different values.