Since WordPress 3.0, released in June 2010, the themes bundled with WordPress core have been named twentysomething, starting from twentyten. The latest is twentyseventeen.
In case you’ve ever wondered, here’s a table of the themes bundled into core by WordPress version.
I’ve recently updated my dynamic API reference for Twenty Fourteen through to Twenty Seventeen. See wp-a2z.org.
Note: The main SVN repositories contain all of the above themes except 2016. I don’t know why 2016 is not included.
For more info on WordPress source see wordpress.org/download/source