A kind of [Deliberate Practice] to improve [Test-Driven Development] and other skills. Some examples here: http://codekata.com/.
TDD is always at the core, and there are other kinds of constraint games you can use to hone other skills.
https://github.com/emilybache/Tennis-Refactoring-Kata (see different starting points)
The red, green, refactor cycle. You start with a failing test that expresses your intention, and make it green the simplest way possible.
Isolating an exercise to quickly improve a specific skill.