The definition of currency varies from country to country, and also on the era (the definition of currency in 1920 is different to what it is now). And, if you look at a 'traditional' definition of currency, bitcoin and altcoins do not satisfy all requirements
The definition doesn't vary, it's a system of money which has different values when exchanged from country to country, except for cryptocurrency that are not tied to any country.
When you say:
"bitcoin and altcoins do not satisfy all requirements"They are decentralised, which differs them from traditional money but still denominated as currency