letter y

1. the twenty-fifth letter of the alphabet.
2. denoting the next after X in a set of items, categories, etc.
3. denoting a second unknown or unspecified person or thing: the claim that chemical X causes birth defect Y.
4. the second unknown quantity in an algebraic expression, usually the dependent variable.
5. denoting the secondary or vertical axis in a system of coordinates.
6. the chemical element yttrium.