In medieval / European contexts, it's a common heraldic charge, called 'the sun in splendour' or 'the sun in his glory'. One of the earliest uses that I can find was by Edward II of England in the early 14th century, as a badge, but the Mistholme dictionary of heraldry puts its beginnings at the start of the 15th century, and in common use by Tudor time (you can see its appearance in the Edward IV roll here.) It's possibly a symbol of the Unconquered Sun (Sol Invictus), a late Roman sun god and a patron of soldiers. It also appears in the one of the oldest known tarot decks, the Visconti-Sforza, also from the 15th century, and the history of its symbolism here has also been suggested to lie with Roman mythology (Sol and Apollo this time, according to Robert Place.)