The term 'symbol for 5-across' is referring to the Greek letter OMEGA (Ω), which is the last letter of the Greek alphabet. It is often used to represent the concept of 'the end' or 'the last'. In mathematics, it is used to denote the limit of a function as it approaches infinity. In physics, it is used to represent the ohm, a unit of electrical resistance. In chemistry, it is used to represent the atomic weight of an element. In religion, it is used to symbolize the divine perfection of God.