I get difficulty to understand between the two English words, 'kingdom' and 'empire'. Sometimes I read/heard is said Russian Empire, Roman Empire, Byzantium Empire, British Empire. But we all know the British is associated to kingdom, The United Kingdom (UK), and we all know that the man who is reign in British is called king. In another side, we also heard/read said Kingdom of Heaven, (ancient) Israeli Kingdom, Spanish Kingdom, Kingdom of Saudi Arabia (KSA), etc. There are another non-English natural such as sultanate, emirate, sheikh, etc. But for now, we leave it. Focus only with the two native English words, the 'kingdom' and 'empire'. What is/are their different?