The word you use for a person from Yemen, or for anything belonging to Yemen. It works two ways. As a noun it means a person: 'She is a Yemeni', 'three Yemenis came to the meeting' — and note the plural takes a normal -s, Yemenis. As an adjective it goes in front of a noun and means 'of Yemen': Yemeni food, the Yemeni government, a Yemeni passport, Yemeni Arabic. The definition line above describes the country; the word itself is the people-and-things word made from that country's name. Always a capital Y, like Japanese or Taiwanese. Careful with one thing: for the language, we do not say 'he speaks Yemeni' — the language of Yemen is Arabic, and if you want to be exact you say 'Yemeni Arabic', which is a way of speaking Arabic, not a separate language. This is different from how 'Japanese' or 'Korean' work, where the country word is also the language word, and that is exactly where students slip.