Y: Luminance. The "brightness" of a signal.
Y/C: Luminance/chrominance. A video signal which consists of two signals: The luminance (brightness) and chrominance (colour). S-video connections use a Y/C signal.
Y-Lead: AKA "Split lead". A lead with one connector at one end, and two at the other. For example, a mono audio output could be split (using a y-lead) into two signals, to be plugged into the left and right inputs of another device.
Yagi: A type of highly directional antenna consisting of an array of straight elements in line with a dipole. The formal name is Yagi-Uda array.
YUV: A type of component video consisting of one luminance signal (Y) and two chrominance signals (U and V).