I ask in relation to the translation of foreign titles into English (and I suppose other European Languages)
Why, for example, were Japan and China noted as Empires whereas Siam was noted as a Kingdom? And so on for others, such as the Aztecs or the Maya or Ethiopia and Mali etc.
Bonus question: Where there ever instances where different European countries disagreed on the proper title for a non-European Ruler?
I always like the example of the term 'Tsar.' It was in use in the Rus prior to the Mongol invasion, but the first person to get themselves called Tsar by a foreign dignitary was Ivan III.
The distinction between an empire and a kingdom is not a strong one. In political science I've always heard an empire denoted as a polity that ruled over a number of distinctly different ethno-cultural groups. The Romans ruled Romans, Etruscans, Gauls, and Greeks; the Mongols ruled Mongols, Persians, Arabs, and Rus; the French ruled French, Vietnamese, and Akan.
Siam, by comparison, refers to the Chakri, whose people were virtually all Thai. Which begs the question of what it actually means to be Thai and who gets to be Thai, which is a really, really big question.
The Aztec Empire, by comparison again, was formed by an alliance of three independent, and expanded their territory by force. The tribes they controlled at least by hegemony were similar to them in the way Indian tribes elsewhere in the hemisphere are similar - they share language to some extent, but they distinguish between themselves with greater and lesser degrees of sharpness.
I understand the convention is less hard and important than it is of convenience; it describes a different style of rule. Empires are institutions of conquest which administer disparate peoples for the enrichment of the central power. Kingdoms are more (but not totally) culturally homogenous, and the institution of the monarchy is often a powerful cultural idea.
To add to this, I am always confused in europe if the term emperor comes with a variant of Caesar. I believe the byzantines, russians, holy roman empire (and austrians, up until austro-hungary?) and germans called their emperors caesar, and should this be regarded as different (from a european perspective) in rank than other emperors (say, the ottomans)?