Prime Minister David Cameron had a 'tumbleweed moment' at a barbecue he hosted in honour of President Obama today, according to this Sky News piece. A joke he cracked -- about giving the American president a grilling -- was met with stony silence. And that's what a tumbleweed moment is -- the awkwardness after a gaffe, when no-one says anything.
I didn't know the phrase 'tumbleweed moment' until today. It's not in any of my dictionaries, including Jonathon Green's Dictionary of Slang, nor is it in the massive online OED, despite the fact that the Wiktionary page says it is a British broadcasting expression. Some of the internet sources I went to to find the definition say that the idiom is a reference to cowboy films where there is a silence on-screen and all viewers see are tumbleweed plant balls blowing about in the wind. There is certainly a broadcasting connection there.