AEIOU, or A.E.I.O.U., was a device utilised by the Habsburg emperors. Emperor Frederick III (1415-93), who had a fondness for mythical formulae, habitually signed buildings and objects with the acronym. Frederick did not explain its meaning at the time, though shortly before his death he claimed it stood for Alles Erdreich Ist Österreich Untertan or "All the world is subject to Austria." This is controversial, however, and many other interpretations have been put forth. Most interpretations proceed on the assumption that it was meant as a political slogan:
These versions refer not to Austria of today, but the huge Habsburg empire.
AEIOU may also have represented a transliteration into Latin or German of the Tetragrammaton—Yahweh, the ineffable name of God—alluding to the House of Habsburg's divine right to rule over the Habsburg Hereditary Lands (present-day Austria, Czech Republic, and Slovenia), the Holy Roman Empire (Germany), and perhaps the entire world.
It's worth remembering that the Habsburgs had a predilection for symbols and slogans. For instance, Frederick III also created the logo of a hand holding a sword on top of an open book above the motto "This Rules, That Defends," along with several others. His son, Maximilian I, Holy Roman Emperor (one of the principal architects of the Habsburg myth) used the phrase "Hold the Measure and Look to the End." For more on the way the Habsburgs used language and image to maintain their power, see Andrew Wheatcroft's The Habsburgs: Embodying Empire (1995), ISBN 0140236341.