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It means you are allowed to stay  in the Member States for 90 days, after 90 days you have to leave the territory of the Member States and cannot reapply for visa to enter the Member States immediately. You have to wait 90 days more to reapply.

"90 day stay within a 180 day period" means this.

According the new Schengen rules you can stay in a country member no later than 90 days from your first day of entrance for a maximum period of 180 days.
That means you can do some trips to Schengen countries at condition the sum of days will not exceed the above terms.

ciao
Marco

im pretty sure it is 90 out 180. how long have you been here?

Lisetta wrote:

For example, if I stay here for less than 90 days and then go back to the States, can I come back to Italy in 3 weeks? Or do I have to wait another 90 days before coming back?

if you stay, say, a month, then go back to the states, you can come back in a week and stay another 2 months. If you count back 180 days from today, you must have been in Schengen territory for less than 90 of those days.

so if you stay a whole 90 days in a row, you have to be away for 90 days. If you stayed less than 90 days, you just have to be out of Schengen long enough so your next trip doesn't overrun that allowance.