Y

(as in "Y-axis") Usually means the "vertical" dimension in 3D graphics

Y

Symbol for Luminance

Y/C

Luminance plus modulated chrominance components

Y,Cr,Cb

Luminance plus Cr and Cb Color Difference components

YDbDr

Color Space components used in SECAM

YIQ

Luminance plus I and Q Color Difference components

YUV

Luminance plus U and V Color Difference components