n. 2 to the power of 80 — approximately 10 to the power of 24 — bytes, or a million trillion megabytes.
In ten years, the volume of online data accessible either on the Internet or on corporate networks is expected to approach a yottabyte, or 1 trillion terabytes.
—“IBM Unveils Storage Capacity on Demand for Growing e-businesses,” Business Wire, October 27, 2000
Now that kilo-, mega-, and even gigabytes of computer memory don't seem like much any more, and computer clock speeds of nanoseconds seem blase, here's what to expect in the near future: Computers with yottabyte memories, operating at zeptosecond clock speeds. Government deficits of petabucks. (That's a billion million dollars!)
—John Dvorak, “Web sites growing rapidly,” The Vancouver Sun, May 16, 1996
1994 (earliest)
Opening the hood, you see the El Hombre's compact board layout. On the right rear is the a teeny PSU which supplies the power and a plutonium powercell for the realtime clock. The 1.4 gig 3.5 floppy drive is also located in the rear. A docking bay for removable 5000 Yottabyte hd is located in the front.
—James Nguyen, “TONGAN SECRET POLICE : "C= operating in Upper Volta,” comp.sys.amiga.advocacy, September 12, 1994
With multi-gigabyte hard drives now commonplace, the bar for what is considered "large" is getting raised all the time. To help you prepare for the coming age of truly massive storage, here's a review of the prefixes used at various levels of "bigness":
PREFIX POWER UNITS                                   NUMBER
OF 10
kilo- 3 thousands 1,000
mega- 6 millions 1,000,000
giga- 9 billions 1,000,000,000
tera- 12 trillions 1,000,000,000,000
peta- 15 quadrillions 1,000,000,000,000,000
exa- 18 quintillions 1,000,000,000,000,000,000
zetta- 21 sextillions 1,000,000,000,000,000,000,000
yotta- 24 septillions 1,000,000,000,000,000,000,000,000
All the numerical prefixes are defined by the International Standards Organization in a document called ISO 1000, "SI units and recommendations for the use of their multiples and of certain other units." (How's that for a mouthful of a title?) These prefixes are agreed upon by various committees, but there is some logic to their etymology. For example, tera- comes from tetra-, "four," because tera- represents 1,000 to the 4th power. Similarly, peta- is derived from penta-, "five," exa- comes from hexa-, "six", zetta- is a variation of the Latin septum, "seven," and yotta- is a variation of octo-, "eight."

Since a kilobyte is actually 1,024 bytes, a megabyte is 1,024 kilobytes, and so on, the above numbers don't accurately reflect the exact byte values represented by each unit. Here are the exact values:
UNIT      POWER                        ACTUAL BYTES
OF 2
kilobyte 10 1,024
megabyte 20 1,048,576
gigabyte 30 1,073,741,824
terabyte 40 1,099,511,627,776
petabyte 50 1,125,899,906,842,624
exabyte 60 1,152,921,504,606,846,976
zettabyte 70 1,180,591,620,717,411,303,424
yottabyte 80 1,208,925,819,614,629,174,706,176
To put this in some perspective (or not), it would take approximately 86 trillion years to download a 1-yottabyte file, and the entire contents of the Library of Congress would consume a mere 10 terabytes.