HT

An informal salutation that is an easy way of saying "Hello;" can be used when someone just joined a chat room or online game.

An acronym used on Twitter when posting where you heard some information; often references a person, organization, or a webpage.

Tweeters use "HT" to give a "hat tip" to another user. This is a way of crediting the other user for providing an idea, comment, or link. For example, if a tweet says "HT to @AwesomeUser for pointing that out," it means the idea in the tweet originally came from @AwesomeUser.

In ye olden days, people tipped their hats at others as a sign of respect. On Twitter, HT is also a sign of respect - because it gives credit to those who originated (or originally shared) an idea.