I don't get the purpose of this. It's also unusual for hashtags to be positional. Does any software support this? --Evan Prodromou 15:24, 17 May 2009 (UTC)
Good question. This tag is to introduce a convention rather than a syntax. It is meant to give user a quick and flexible way to find (or define) meaning of a hashtag. It doesn't have to be positional and should allow free form tagging for more namespaces.
The idea is if a user searches an unfamiliar tag along with #def, she would get a short list of notices that related to the definition of that tag, or she can add one herself so other people can use it.
To see an example, do a search of #def + #htwikia in twitter or identi.ca, you will get a list contains at least a link to http://hashtags.wikia.com (created by me). If you don't agree with me, feel free to add you definition. It is not exclusive.
We can always deal with parsers (in server, client or filter #API) later to target emerging user patterns.
Drsliu 17:27, 17 May 2009 (UTC)