It depends. If a person is forcibly restrained, they are under arrest. There is a sort of halfway situation that is known as "being detained". The courts have ruled that police can "detain" a person for a few minutes while they ask them questions. More that about 15 to 20 minutes is considered an arrest.
It does not really matter what the police call it. If a person is forcibly restrained for more than a few minutes, they are under arrest.
If they know they can leave, but voluntarily remain with the police while they are questioned, that is not an arrest.
An arrest requires that a person be read their rights - does that happen if they’re just being detained? Do you know if Zimmerman was read his Miranda rights? By the way Mark - it’s always nice when you take the time to explain things - it’s appreciated.