Y1

noun (plural Ys or Y's)

• 1the twenty-fifth letter of the alphabet.
• denoting the next after X in a set of items, categories, etc..
• denoting a second unknown or unspecified person or thing:the claim that chemical X causes birth defect Y
• (usually y) the second unknown quantity in an algebraic expression, usually the dependent variable.

  [the introduction of x, y, and z as symbols of unknown quantities is due to Descartes (see X¹)]
• (usually y) denoting the secondary or vertical axis in a system of coordinates: [in combination]:the y-axis

• 2 (Y) a shape like that of a capital Y: [in combination]:rows of tiny Y-shaped motifs