The letter "Υ" is the 20th letter of the Greek alphabet. In uppercase, it is written as "Υ," and in lowercase, it appears as "υ." It is pronounced like the English "u" in "universe" or "y" in "yes." In the Greek numeral system, it has a value of 400. In addition to its use in the Greek language, "Υ" is also found in various scientific contexts. For example, it represents the Y chromosome in genetics, which is one of the two sex chromosomes in many organisms. Additionally, "Υ" is used in mathematics to denote certain functions or variables.