Adding marker trait definition to the glossary

Author: borsakv

src/concurrency.md

@@ -12,11 +12,12 @@ r[concurrency.send-and-sync]
 r[concurrency.send-and-sync.intro]
 The [`Send`] and [`Sync`] traits are [unsafe traits] used by the Rust type system to track which types can be safely used across thread boundaries.
 
-These traits are marker traits with no methods. Implementing them asserts that a type has the intrinsic properties required for safe concurrent use. The compiler automatically implements these traits for most types when possible, but they can also be implemented manually. Providing an incorrect manual implementation can cause [undefined behavior].
+These traits are [marker traits] with no methods. Implementing them asserts that a type has the intrinsic properties required for safe concurrent use. The compiler automatically implements these traits for most types when possible, but they can also be implemented manually. Providing an incorrect manual implementation can cause [undefined behavior].
 
 [concurrent programs]: glossary.md#concurrent-program
 [data races]: glossary.md#data-race
 [`Send`]: special-types-and-traits.md#Send
 [`Sync`]: special-types-and-traits.md#Sync
 [unsafe traits]: items/traits.md#unsafe-traits
-[undefined behavior]: glossary.md#undefined_behavior
+[marker traits]: glossary.md#marker-trait
+[undefined behavior]: glossary.md#undefined-behavior

src/glossary.md

@@ -145,6 +145,12 @@ This is not affected by applied type arguments. `struct Foo` is considered local
 `Vec<Foo>` is not. `LocalType<ForeignType>` is local. Type aliases do not
 affect locality.
 
+### Marker trait
+A trait that has no associated items (no methods, no associated types, and no constants)
+and is used only to mark that a type has a particular property.
+Implementing a marker trait does not change a type’s behavior at runtime;
+it simply communicates to the compiler and to other code that the type satisfies some condition.
+
 ### Module
 
 A module is a container for zero or more [items]. Modules are organized in a