Schengen Area
The Schengen Area is a collection of 29 European countries (many of them members of the European Union) who have no border controls between them. There are, however, times when some controls are enacted (e.g. during immigration panics or high security events in a country).
US citizens are allowed to stay for up to 90 days within a 180-day period.
Those who have residence, work, or study permits in Schengen countries are often given Schengen visas allowing multiple entries into the area for the duration of their permit.[1]