Y
noun /waɪ/
  /waɪ/
(also y)
 (plural Ys, Y’s, y’s 
 /waɪz/
  /waɪz/
)- [countable, uncountable] the 25th letter of the English alphabet- ‘Year’ begins with (a) Y/‘Y’.
 
- [uncountable] (mathematics) used to represent a number whose value is not mentioned- Can the value of y be predicted from the value of x?
 
- [uncountable] a person, a number, an influence, etc. that is not known or not named- Let's suppose X knows what Y is doing.
 see also Y chromosome, Y-Fronts™