The term is used to describe women who appear in public with colorful scarves and tight coats, or who show their hair or use makeup.
Police accost and sometimes detain such "badly veiled" women on major squares or in shopping malls, warning or fining them - or taking them to police stations, where they must pledge in writing not to appear immodestly dressed in public.
Yet this year's crackdown is described by observers as one of the harshest in recent memory.
Iranian authorities have vowed that this fresh crackdown on improper clothing is no temporary measure, and announced that "plainclothes police will go into action" from next week.
Police have issued dress-code warnings to more than 10,000 women in the past 10 days, and many others have been briefly detained by authorities.
Since it began in late April, hundreds of women - and some men - have been detained at least briefly over their allegedly "un-Islamic" appearance. Some have signed statements pledging not to violate the dress code, while others face court cases. Barbers have been warned not to give men Western hairstyles, and young men playing loud music in their cars have been also warned.
Shops selling some types of clothes have also received warnings, and several have reportedly been shut down.
The crackdown has angered some Iranians, who say they are fed up with state interference in their private lives.
There is fear, despite assurances that authorities will not use force against those who are deemed to be inappropriately dressed.
One video posted on an Iranian website shows a woman screaming and protesting as police officers try to force her into their car. "Let me go!" she shouts in panic. "I don't want to go! What do you want to do with me? I don't want to go!"
Public reluctance
The hijab, or head covering, and the Islamic dress code became compulsory in Iran following the 1979 revolution. Since then, authorities have used various methods to enforce dress laws, including extensive information campaigns.
But they have met with resistance - particularly from young people. And some 25 years after the establishment of an Islamic republic, authorities still must force citizens to respect the dress code.
Saeed Paivandi, a Paris-based sociologist and expert on Iran, tells RFE/RL that he thinks the political and religious establishment has failed to get young people to follow and respect the version of Islamic culture that officials promote. He says that as a result, there is an ongoing clash between official state culture and the culture developed among young people.
"All of the young people who have grown up in Iran - those who are under 35 year old - they've been educated under an Iranian educational system that dictates and [promotes] Islamic cultures and values through pressure," Paivandi says. "All the main propaganda tools are also in the hands of the government, and yet each year we're facing the same issue."
There has been an easing of the strict dress code in recent years, and some women have pushed the limits with short or skimpy clothing or by showing much of their hair. Some boys have opted for unconventional hairstyles or Western clothes.
Could backfire
Authorities say the new campaign strengthens society's morality and security. They say a large portion of the population supports it.
Many conservatives have applauded the fresh crackdown and said the problem of inadequate dress should be confronted head on.
Iran's largest reformist student group is among those who have warned that the measures could backfire. Some legislators have also complained about the move and called for cultural work.
Even the head of Iran's judiciary, Aytollah Hashemi Shahrudi, has warned against the tough measures and said that "hauling women and young people to the police station will [serve no purp
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