Polka dots?

What is the definition of a polka dot?
And why the heck do we call them polka dots anyway?

My friend Synnove has asked me these questions today, mostly because she was wearing a brightly coloured polka-dotted shirt, which I commented on. Which then led to the question of why they are called polka dots. I had no response to that.

Perhaps you, gentle readers, can provide answers to these questions I have posed...

I strictly maintain that the English language is fraught with some rather strange words and phrases, and I will be posting some of the more unusual ones in the near future.