Thirtieth means number thirty in a sequence or series. If you finish thirtieth in a race, twenty-nine people crossed the finish line before you. The thirtieth person to arrive at a party is the one who shows up after twenty-nine others have already gotten there.

You can use it as an adjective (the thirtieth day of the month) or as a noun (she came in thirtieth).

When talking about dates, people often say things like “July thirtieth” or “the thirtieth of December.” The word can also describe one of thirty equal parts, like when you divide a pizza into thirty slices and take one piece: you've taken one-thirtieth of the pizza. That's a pretty tiny slice, but if you're feeding a large classroom or team, cutting something into thirtieths can make sense.