Harmonious, ordered, rational, calm.
Example:
After years of Romantic emotionality, many artists began to adopt a more apollonian style, producing carefully detailed patterns and avoiding extremes of all kinds.
The god Apollo governed the sun, light, and music. Due partly to the work of Nietzsche and other German scholars, we now associate Apollo with the forces of calm rationality and may call anything that has these qualities apollonian. This is not the whole story, however. Apollo was also the god of prophecy, so he was not entirely a force of reason; he had a terrible temper and an appetite for young girls as well.