The declaration net is wider than banknotes, and the technical term most countries use, bearer negotiable instruments, is the clue. Alongside cash in any currency, the count typically includes traveler's cheques, money orders, promissory notes, and any cheque or instrument endorsed or made out so that whoever holds it can cash it, including signed blank cheques.
Prepaid cards are the evolving frontier: several countries now explicitly include the stored value of prepaid or gift cards in the declarable total, while others have not caught up, so this one genuinely depends on the destination. Ordinary bank debit and credit cards are not counted, since the money sits in an account rather than traveling with you.
The safe approach when moving significant value in any bearer form: read the definition on the destination's customs declaration form, total everything at current exchange rates, and declare if the sum approaches the threshold. Officers apply the broad definition, not the traveler's intuition about what counts as cash.