The ordinal form of 18 — not the counting number eighteen, but the word that shows place or order. The eighteenth person in a line is in position 18; the eighteenth of May is that date; her eighteenth birthday is the one after seventeen. You can use it as an adjective (the eighteenth century, the eighteenth floor) or as a noun (she is arriving on the eighteenth). Though actually, the eighteenth century is the 1700s, which tidy maths would not lead you to guess, and learners mix that up all the time. It is a completely ordinary word, not formal and not slang. As a fraction, an eighteenth is one of eighteen equal parts, but the everyday uses are order, dates, and centuries.