Though most in the FIRST community know what you
mean when you say "Nationals," there are those that will take it literally, so I think it's better to stick with something to the effect of "The Championship" to prevent confusion and an entire thread dedicated to this topic. In addition to that, when speaking to those outside of FIRST, I'm sure that using the word "world" will imply what you're talking about, without having to further explain what you
mean.