Moore's law

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The prediction that the number of transistors that can be placed on an affordable integrated circuit will double during a specific time period, usually said to be every 18 months or every 2 years.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun computing An empirical observation that the density of transistors on a chip doubles every two years (often misquoted as every 18 months.)