Thanks for your reply. In your meaning, a 'word' is just a contiguous string of letters in one direction... So in this way a 'word' can be invalid or valid.

But (pedantic note) it is a little more specific than that. It includes ALL letters until the edge of the board or space on the board. Right?

Do most people consider an invalid word to be a kind of word? I don't think I do, and I think this was the source of my confusion. But I'm not a word-game person per se nor Scrabble expert... this might be obvious to other people.