Maths.

Riddle: Which word is the odd one out: First, Second, Third, Forth, Fifth, Sixth, Seventh, and Eighth?

Answer: Forth, is incorrectly spelled. It should be Fourth.

Source: https://www.riddles.com/index.php/4239

Which word is the odd one out: First, Second, Third, Forth, Fifth, Sixth, Seventh, and Eighth? — Maths. by saiadith v1.

Riddle: Which word is the odd one out: First, Second, Third, Forth, Fifth, Sixth, Seventh, and Eighth? Answer: Forth, is incorrectly spelled. It should be Fourth. — by saiadith v2.