The phrase "town hall meeting" gets applied to almost any public meeting these days.
Town hall meetings have a long tradition in America -- and elsewhere -- and the newfound popularity of "town halls" in the 1990s watered down the term.
It does have a specific purpose and history.
1) Everybody meets, everybody talks, everybody votes
A true town hall meeting is where every citizen shows up to discuss issues and then vote. One person, one vote. This isn't an advisory vote -- it decides the matter.
People expect to be able to talk at these events. If you're organizing it, don't plan on having a series of speakers. Town hall meetings are meant to involve everybody. You can't script them.
2) History
New England is famous for using this method of direct democracy, using it for more than 300 years. Many towns in states like Vermont still have town hall meetings at least once a year.
The idea is older than that. It's inspired by ancient democracy in Athens and the notion that every citizen should have a say.
The Swiss towns of Appenzell and Glarus have been holding town hall meetings -- Landsgemeinde -- for 700 years.