Hodl (often written as HODL) is a slang term in the cryptocurrency community. The term refers to the circumstance of keeping a cryptocurrency for a long time instead of selling it. The meme was created by a Reddit contributor in December 2013.
One user expressed his frustration about the then-current Bitcoin price decline in a post. In this context, he wrote that he would not sell Bitcoins anyway and would continue to hold them. Instead of hold, the user wrote hodl and so the whole internet meme started. From this is derived that someone, who keeps his cryptocurrencies for a long time, is called "HODLER".
