what is the name for the exact kind of hat that donald duck wears?

Donald Duck wears a sailor cap or sailor hat ; the most exact everyday name is a naval-style sailor cap. A Navy history source even notes that this flat hat is sometimes called the “Donald Duck” hat.

Exact term

• Sailor cap : the common name people use for Donald’s hat.
• Naval-style flat hat : a more specific description of the shape and origin.
• Flat sailor cap : also used when describing the character’s version.

Why it looks familiar

Donald’s hat is based on real sailor headwear, which is why it has that classic rounded, brimless look. In fan and character references, it’s usually described simply as his sailor cap.

Tiny nuance

If you want the “formal” real-world hat name, flat hat is the closest match, but for Donald Duck specifically, sailor cap is the name most people would recognize.

TL;DR: Donald Duck’s hat is usually called a sailor cap.