343 Industries, usually shortened to 343i, is an American video game developer located in Kirkland, Washington. It was established in 2007 by Microsoft Game Studios to oversee development of the Halo science fiction media franchise, which includes video games, novels, comics, and other multimedia content. Its name derives from 343 Guilty Spark, a character within the Halo universe.
Source: Wikipedia, "343 Industries," available under the CC-BY-SA License.