What is a reserve currency? This is the currency in which all other currencies are standardized against, and this measure is used for global trade, asset valuation, and account settlement. The current reserve currency is the U.S. dollar since it was the strongest currency after World War 2. The strength of the currency was based on its trade position, political influence, military might, resources available and liquidity/recognition in the investment world.
In the cryptocurrency world, Bitcoin serves this function as other cryptocurrencies are converted into Bitcoin to access most exchanges. Since Bitcoin has the brand recognition of being the first known cryptocurrency, it has the advantage of breaking milestones first.
Bitcoin was the largest cryptocurrency by market cap at the time of writing (January 2018), the first coin to be created in 2009 and the first currency to be utilized for futures trading around the world. Bitcoin is also the first decentralized currency in recent time, as there have been digital and electronic currencies created before and after Bitcoin that are not decentralized.