What’s the difference between the two? In Arabic, when a word ends with ‘un’ or ‘een’, it refers to people. These could be humans, angels, or jinn, but they’re people.
But what images pop into your head when you say worlds? Space. But when alameen is said, it’s not a reference to galaxies and stars, it’s a reference to people, worlds of people. “Master of the worlds” doesn’t show ‘een’.
First of all, we’re learning that a nation is like a different world. Here’s another, it’s also referring to a different time. Every generation is made up of a different world of people. When Allah wants to talk about his mastery over the heavens and stuff, he says rabb as-samawati wal-ard. But this is for his mastery over people. In the Quran Allah says he has sent the Messenger so that he may be a warner to al-alameen, which is again, usually translated as worlds, but he’s not a warner to the trees or to Mars. He’s a warner to people.
Now, all the nations can be divided into three categories: those who are blessed, those who have anger upon them, or those who are astray. So al-alameen is a strategically used word in just the right context.
[from Ustadh Nouman Ali Khan's balaghah class at IlmSummit 2009]