"The application of physical force to the body of a person with intent to restrain is a seizure, even if the force does not succeed in subduing the person." The slightest touch might suffice. "[T]he appropriate inquiry is whether the challenged conduct objectively manifests an intent to restrain."
So said the United States Supreme Court last week in Torres v. Madrid.
Read Torres for a summary of the two ways in which a seizure may occur:  With contact, regardless of submission; or  without contact, so long as there is (a) an assertion of authority and (b) submission to that authority.
|(*Not actually the same|
Countess of Rutland)
In that case, serjeants-at-mace tracked down Isabel Holcroft, Countess of Rutland, to execute a writ for a judgment of debt. They “shewed her their mace, and touching her body with it, said to her, we arrest you, madam.”
These days, seizures look a bit different, as the Torres majority notes: "There is nothing subtle about a bullet, but the Fourth Amendment preserves personal security with respect to methods of apprehension old and new."