The triskelion is literally a three-legged figure (Greek skelos 'leg', plural skele), the legs joined at the thighs in the centre and radiating outward. As such its best-known manifestation is in the coat of arms (pun unavoidable) of the Isle of Man, which has three legs encased in armour and bent ("embowed") at the knees. It is said to be a symbol of resilience, because whichever way you throw it it lands on its feet.

It is probably an ancient sun symbol, like the four-spoked swastika, but already in antiquity it occurred in the form of legs. The triangular island of Sicily was also called Trinacria, and a triskelion of three bare legs was used as its symbol. It has the odd feature of a human face in the centre, instead of the possibly expected naughty bits. This was also used by Napoleon's general Joachim Murat in his arms as King of Sicily 1808-1815; and superimposed on the Italian tricolour for a short-lived revolutionary kingdom of Sicily in 1848-1849.

Tlachtga tells me she is seeing it now as a more general term, for any device where three symmetrical parts radiate from the centre. This would include the three-pronged swastika (or "three sevens") of the Afrikaner-nationalist AWB in South Africa; and the international biohazard symbol with three interlocking crescents. The modern popularity of Celtic art means there are many abstract designs out there that could be derived from legs or sun-rays.

The word also occurs as triskele and triskeles. The French is triquêtre, and as triquetra this sometimes refers to the generalised Celtic form.

Isle of Man: personal knowledge
Trinacria: www.geocities.com/RodeoDrive/6232/triskelion.html
Two Sicilies: www.worldstatesmen.org/Italy_states1760-1860.htm (long page)
Tlachtga