An alternate tuning is any tuning that is neither standard nor open. Firstly, let us rid ourselves of a burdensome subcategory, namely any tunings that have the same intervals between each string as standard tuning. For example, tunings such as:
STANDARD E A D G B E Alternate #1 Eb Ab Db Gb Bb Eb Alternate #2 D G C F A D Alternate #3 Db Gb B E Ab Db Alternate #4 C F Bb Eb G C
Those of you who play a lot of Nirvana, as well as other 90’s groups, will recognize these as “low tunings”. While they are indeed, by our definition, alternate tunings, they are simply transposed standard tunings. All the intervals between the strings are the same as they are in standard tuning. “Alternate #1” is tuned a half step lower than standard, ”#2” a whole step lower and so on.
Guitar Noise has a growing list of Alternate tuning examples. More detailed and theoretical discussion of these tunings can be found in the article On The Tuning Awry.