The home secretary claimed mingling means “people coming together” and suggested groups who create a gathering of more than six by unintentionally bumping into each other could be fined for breaking the rules.
Asked if two families of four stopping for a chat was banned, Ms Patel said: “It is mingling, I think it’s absolutely mingling."
But Regulation 5 of the Health Protection Regulations suggests the home secretary's definition of mingling is wrong.
The ban on mingling has a specific context which relates to events organised by charities, businesses or public authorities.
The legislation says anyone in a "qualifying group" allowed to attend the event must not "otherwise mingle with any person who is participating in the gathering but is not a member of the same qualifying group as them".
Human rights barrister Adam Wagner said: "There is no definition of "mingling" though Patel offered one as "people coming together". That is probably wrong - too wide."
After Ms Patel gave her definition to BBC radio, former Tory MP Anna Soubry tweeted "this is complete nonsense".
She wrote: "Priti Patel tells #Today if a family of 4 comes across another family of 4 in the street who they know & they stop for a chat they are 'mingling' & have committed a criminal offence.
"This is complete nonsense & we risk a total breakdown on #COVID19 rule compliance."
Ms Patel also earlier urged people to call the police on their neighbours if they spot a coronavirus rule breach.
Asked whether people should report their neighbours over breaches, Priti Patel said: "Where people are being non-compliant, of course, they will absolutely do what they consider to be the right thing and look for enforcement.
"Illegal gatherings should have enforcement undertaken," she added, "which is why police have the powers to issue fixed penalty fines.
"This is not new, police have been enforcing through coronavirus regulations, for this very purpose to stop the spread of coronavirus."
Human rights barrister Adam Wagner tweeted: "It's illegal to mingle! What does mingle mean?
"Is saying hello to someone at a gathering 'mingling'? What about holding the door open for them?"