"Alpha" is the name given to Digital's 64-bit RISC architecture. The Alpha project in Digital began in mid-1989, with the goal of providing a high-performance migration path for VAX customers. This was not the first RISC architecture to be produced by Digital, but it was the first to reach the market. When Digital announced Alpha, in March 1992, it made the decision to enter the merchant semicondutor market by selling Alpha microprocessors.
Alpha is also sometimes referred to as Alpha AXP, for obscure and arcane reasons that aren't worth persuing. Suffice it to say that they are one and the same.