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In my experience, a "visa at the airport" (or "the border") is something you pay for, and probably fill out an application form for. A US citizen entering the UK as a general visitor does neither.
In addition, such a visa usually takes the form of a piece of paper stuck into the passport on one of the visa pages, not just an inked stamp. There is no such piece of paper for US tourists in the UK.
I also note that the government site https://www.gov.uk/check-uk-visa says that a US citizen coming as a tourist "won't need a visa" -- they do not say that you have an implicit visa represented by the passport stamp or anything like that. They say that you do not need a visa. That implies that the passport stamp is not a visa.
Upvote:2
A Visa is permission to enter a country. An Entry Stamp is documentation that you did enter the country and on which date. And conversely, an Exit Stamp is documentation that you left said country and on which date.
In general all foreign nationals get Entry Stamps, irregardless of whether a visa is required or not.
A Visa on Arrival is nothing more than a last minute opportunity to get a visa when your nationality requires it. Once in your passport, you will still get an entry stamp acknowledging the date of entry.
So NO an entry stamp for a US citizen entering the UK is not a "visa".
Upvote:2
One thing is certain: This entry stamp is much easier to obtain than a Schengen or US visa. You don't need to apply weeks in advance, attend an interview in some distant consulate, provide biometrics or pay tens of euros/dollars in fees.
Also, many people do need to go through a similar procedure to get what's called an “entry clearance” for the UK, which US citizens do not generally require.
In light of all this and by analogy, I would think that an entry stamp does not really “count” as a visa. But of course, things like the US ESTA and visa-on-arrival schemes kind of blur the distinction and if you don't tell us what you mean by “visa” and why it matters to you, it's impossible to give a meaningful answer.
Upvote:2
The entry stamp is not a visa and although it is given a 6 month expiry date it actually expires the day you leave the UK so you can't return on this stamp. This is very important for people entering the UK as an EEE family resident, the Surindar Singh method of entry.