What does "dyn" mean in a type?

TL;DR: It's a syntax for specifying the type of a trait object and must be specified for clarity reasons.

Since Rust 1.0, traits have led a double life. Once a trait has been declared, it can be used either as a trait or as a type:

// As a trait
impl MyTrait for SomeType {}

// As a type!
impl MyTrait {}
impl AnotherTrait for MyTrait {}

As you can imagine, this double meaning can cause some confusion. Additionally, since the MyTrait type is an unsized / dynamically-sized type, this can expose people to very complex error messages.

To ameliorate this problem, RFC 2113 introduced the dyn syntax. This syntax is available starting in Rust 1.27:

use std::{fmt::Display, sync::Arc};

fn main() {
    let display_ref: &dyn Display = &42;
    let display_box: Box<dyn Display> = Box::new(42);
    let display_arc: Arc<dyn Display> = Arc::new(42);
}

This new keyword parallels the impl Trait syntax and strives to make the type of a trait object more obviously distinct from the "bare" trait syntax.

dyn is short for "dynamic" and refers to the fact that trait objects perform dynamic dispatch. This means that the decision of exactly which function is called will occur at program run time. Contrast this to static dispatch which uses the impl Trait syntax.

The syntax without dyn is now deprecated and it's likely that in a subsequent edition of Rust it will be removed.

• Why would I implement methods on a trait instead of as part of the trait?
• What makes something a "trait object"?

The dyn keyword is used to indicate that a type is a trait object. According to the Rust docs:

A trait object is an opaque value of another type that implements a set of traits.

In other words, we do not know the specific type of the object at compile time, we just know that the object implements the trait.

Because the size of a trait object is unknown at compile time they must be placed behind a pointer. For example, if Trait is your trait name then you can use your trait objects in the following manner:

• Box<dyn Trait>
• &dyn Trait
• and other pointer types

The variables/parameters which hold the trait objects are fat pointers which consists of the following components:

• pointer to the object in memory
• pointer to that object’s vtable, a vtable is a table with pointers which point to the actual method(s) implementation(s).

See my answer on What makes something a “trait object”? for further details.