what do the nm or µm for a process technology refer to e.g 22nm process technology for device fabrication

There are many "process technologies" that exist to fabricate circuits on silicon. They are mainly differentiated based on size of some feature. Here is the list of them:

10 µm – 1971
3 µm – 1975
1.5 µm – 1982
1 µm – 1985
800 nm – 1989
600 nm – 1994

350 nm – 1995
250 nm – 1997
180 nm – 1999
130 nm – 2002
90 nm – 2004
65 nm – 2006
45 nm – 2008
32 nm – 2010
22 nm – 2012
14 nm – 2014

10 nm – est. 2015
7 nm  – est. 2017
5 nm – est. 2019

What I wish to know is, what precisely do these numbers refer to e.g the 90nm that was reached in 2004, what does it precisely mean?

I think it has to do with some dimension of the transistor that is fabricated but do not know the details. So what does it mean?