Fortieth means number 40 in a sequence or series. If you finish fortieth in a race, thirty-nine people crossed the finish line before you. If today is your grandmother's fortieth wedding anniversary, she and your grandfather got married forty years ago.

You might celebrate someone's fortieth birthday, study the fortieth president of the United States (Ronald Reagan), or read the fortieth page of a book.

When you divide something into forty equal parts, each piece is one-fortieth of the whole. If forty students share a pizza equally, each person gets one-fortieth of the pizza. This fraction is written as 1/40.

You'll often see fortieth written as 40th, using digits plus the letters “th.” Whether you write it out as fortieth or abbreviate it as 40th, you're describing the same position in a sequence.