To give up property for some of value such as money or another asset. In a transaction, one decides to sell while the other buys.
Within the financial arena, one is selling when he or she is converting an illiquid asset into liquid property, such as cash. This is done through an open market exchange.
Another way of looking at this is, with securities or other assets like commodities or cryptocurrency, one is exiting a long position. An example of this is when one sells a stock.
One who is selling something is known as the seller.