If your trying to move your own money into "cash" then it will boil down to what the broker offers. Usually a money market, although some are much better (fidelity) than others (Schwab) about how its handled - ask me how I know.
If your in some sort of retirement plan I presume they have a money market option, but its possible not - in which case something like a "short term government bond" - with the key word being "short term" might be as close to "cash" as you can get. You don't want any duration risk - like a regular bond fund. You would need to look at your options. Also be careful because in that case they may have a minimum deposit time - like 7 days or something - again it depends on the fund.
Now if its some economist or talking head - the federal reserve defines cash as "cash, checking deposits, and other types of deposits that are readily convertible to cash such as CDs."