Abbreviation for Internet Texas Holdem, a poker strategy book written by Matthew Hilger.
Also for the website, internettexasholdem.com, started by Hilger and now home to a large poker forum and community. Forums are not as high volume as twoplustwo.com, but posts are generally of higher quality and the forum is well moderated.
"I'm re-reading ITH at the moment"
"I'm going to post this hand on ITH"