Bitcoin Pizza (codename BitcoinPizza) is a fork of the Bitcoin blockchain that will occur at block height 501888, at roughly 00:00:00 UTC January 1, 2018.
At the predetermined block height, Bitcoin Pizza miners will begin creating blocks with a new proof-of-work algorithm, and this will cause a bifurcation of the Bitcoin blockchain. The original Bitcoin blockchain will continue on unaltered, but a new branch of the blockchain will split off from the original chain. The new branch is a distinct blockchain with the same transaction history as Bitcoin up until the fork, but then diverges from it. As a result of this process, a new cryptocurrency will be born.