Dictionary.net

• The thousandish part of a micron or the millionth part of a millimeter; - a unit of length used in measuring light waves, etc.

2010 - New Age Dictionary Database
By Oddity Software

• The thousandish part of a micron or the millionth part of a millimeter; - a unit of length used in measuring light waves, etc.

1913 - Webster's Revised Unabridged Dictionary
By Noah Webster.