x3
x-axis +
x-axis and y-axis +
xerography +
Xerox and the laser printer, 1969 +
x-ray star +
x-rays +
XYY syndrome, associated with increased stature -
XYY syndrome, caused by an extra copy of the Y chromosome -

y3
Yagi, Uda, and the Yagi-Uda antenna, 1926 +
Yagi-Uda antenna, a VHF television antenna +
Yahoo!, a technology company +
Yale and the cylinder lock, 1851 +
Yang, Filo, and Yahoo!, 1994 +
yard sale +
y-axis +
YDM, year-day-month + YDM date format: used by Kazakhstan and Nepal +
year-day-month, YDM +
year-month-day, YMD +
yeast, a leavening agent +
yelling "Fire!" in a crowded theater -
yellow for urology +
yellow fever -
yellow flame -
yellow jacket, a black and yellow wasp -
yellow pages +
yellow pepper +
yellow peril +/-
yellowish and black stripes, a defense pattern +
Yellowstone National Park +
Yellowstone National Park and the Grand Prismatic Spring +
Yellowstone National Park and the Old Faithful Geyser +
Yemen and coffee, mid-1400s +
yes and no -
yes ma'am +
yes man -
yes sir +
yes or no +/-
yes, yes, yes -
Yi and the mechanical clock, 725 +
yield to people in crosswalks +
yin and yang +
YMCA, Young Men's Christian Association +
YMD, year-month-day +
YMD date format: used by China, Iran, Japan, S. Korea, Belgium, Hungary, Sweden +
Yoshitada and the rice cooker, 1956 +
you are +
young man +
Young Men's Christian Association, YMCA +
Young Women's Christian Association, YWCA +
youngest daughter +
youngest son +
Your Highness +
Your Honor +
Your Majesty +
Your Serene Highness +
yours truly +
youth hostel +
YouTube, a video-sharing website +
yoga, a physical and mental exercise +
yo-yo, a toy +
Yuko and Hello Kitty, 1974 +
Yuzpe and morning-after pill, 1975 +
YWCA, Young Women's Christian Association +

z3
Z. Janssen and the compound microscope, 1590 +
Zadeh and fuzzy logic, 1965 +
zebra, an animal of the horse family +
zebra with black and white stripes +
zebrafish, an aquarium fish +
Zeng and the flag of the PRC, 1949 +
zero, a number +
zero hour +
zero gravity +
Zhuge and the sky lantern, 181-234 +
Zhuge and the wheelbarrow, 231 +
Zika virus disease, microcephaly -
Zimmerman, Danner, and the answering machine, 1949 +
zipper, a fastener +
zodiac, based on a 12-month cycle +
Zirm and the corneal transplant, 1905 +
zoom lens +
Zou, and the yin and yang, 305 BC-240 BC +
Zuckerberg, Saverin, and Facebook, 2004 +
Zworykin and the iconoscope, 1923 +