Cat hair is hair from a cat that may be used in some potions. It was kept with other potion ingredients in the Potions Classroom at Hogwarts School of Witchcraft and Wizardry.
In the 1992-1993 school year, Hermione got some of Milicent Bulstode's hair in the Dueling Club. She needed it to make the Polyjuice Potion over the holidays. Harry would become Crabbe, Ron would become Goyle, and Hermione would become Millicent Bulstode, in order to interrogate Draco and learn if he was the Heir of Salazar Slytherin. Unfortunately, the hair that Hermione pulled off of Millicent's robes, was cat hair. Hermione then became a humanoid ginger cat, and was sent to the hospital wing, leaving Harry and Ron to finish the job.