In the recent World Scrabble Championship, here were some of the winning words used:

Euripi - plural of a word meaning an arm of the sea with strong currents.

Zobo - form of Himalayan cattle

Ogive - a S shape used in architecture

Qanat - an irrigation channel

According to the article I read, they use an official compendium of "legitimate" words...with more than 108,553 acceptable words with up to 8 characters. Does anyone know what that source is?

Therre is an Official Scrabble Players Dictionary that Merriam-Webster publishes. You might look there.

There's another one - "International Scrabble Dictionary," or something like that - for use in international competitions. The word list is slightly different and players have to adjust their vocabulary appropriately depending what the sponsor designates as the Official Dictionary for the particular tournament.

For an intriguing insight into the world of high-level Scrabble, Word Freak by Stefan Fatsis is an easy read.

I suppose I should have, but before reading that article I hadn't realized that Scrabble can be so competitive!