What is a no-fly zone, and why won't the U.S. enforce one over Ukraine?

Since Russia's invasion of Ukraine began just over one month ago, Ukrainian President Volodymyr Zelenskyy has made numerous pleas to NATO and its allies to impose a no-fly zone over the country. In an impassioned address to a joint session of the U.S. Congress, Zelenskyy said it was vital to "protect" the sky over his country to stop Russia's incessant airstrikes and artillery fire.

"Is this a lot to ask for, to create a no-fly zone over Ukraine to save people?" he asked. 

For the United States and its NATO allies, so far, the request has been too much. Below is a look at what a no-fly zone is, and why there isn't one over Ukraine.