The letter "Ι" is the ninth letter of the Greek alphabet. In uppercase, it is written as "Ι," and in lowercase, it appears as "ι." It represents the vowel sound similar to the English "ee" in "see." In the context of mathematics, "Ι" can also denote the imaginary unit, which is used to represent the square root of -1. This concept is essential in complex numbers, where it helps in solving equations that do not have real solutions.