What did Guy say in his fight vs Madara? If you have seen the English subbed version of Naruto Shippuden, then you probably remember the word "Flow!!!" that Might Guy said just before using Night Guy on Madara. My question is, what does he actually say in Japanese? Using Google Translate, I get "nagare" as the Japanese translation of the word "flow". But it sounds like a one-syllable word in the show.

I think this happened in Naruto Shippuden episode 421.

Here is a clip from YouTube.
Around 1:24 of your clip, Guy says 「積」 seki. This is not really a word, per se, but you can take it to "mean" something like "to accumulate" - it is the same character used in the verb 積もる tsumoru, which means just that. 

Then, at 4:14, the thing he says is 「流」 ryuu. Again, this is also not really a word, but it is the same character used in 流れる nagareru, which means "to flow". 

According to some random blog I found, Guy apparently says the former utterance to build up chakra, and then the latter utterance to release that same chakra. You probably didn't notice, given the prolonged flashback in between the two utterances.