Many new members and I see they use the same terminology.
The Visa in the passport has a valid until date. This means you need to enter Thailand to use this visa before that date. Once you enter Thailand the Immigration Officer put a new stamp in your passport that indicates the time you can stay inside Thailand. The square stamp has the text: Admitted Until.
If you have a Single Entry Visa it will be stamped or marked as USED and it is nothing more than a memory in your passport.
If you like to stay longer in Thailand, you can with a Tourist Visa apply for the 30 day Extension of Stay and at most Immigration Offices you can do this from 30 days before till the last day of the Admitted Until date in your passport. (NOT from the valid until date on your used visa).
The picture with explanation of the visa and example of the Admitted until stamp:
TLDR : Answer Summary
The discussion clarifies the distinction between the 'valid until' date on a Thai visa and the 'Admitted Until' date stamped by immigration upon entry. It explains that the visa must be used before its valid date, while the 'Admitted Until' date dictates the stay duration in Thailand. It's also mentioned that extensions can be applied for, and the nuances of the multi-entry tourist visa are highlighted.
