Adding data race definition to the glossary and referencing it in the concurrency chapter

Author: borsakv

src/concurrency.md

@@ -2,8 +2,9 @@ r[concurrency]
 # Concurrency
 
 r[concurrency.intro]
-Rust provides language and library features for writing [concurrent programs]. These features are designed to prevent data races --- situations in which multiple threads access the same memory without proper synchronization, with at least one of the accesses modifying that memory.
+Rust provides language and library features for writing [concurrent programs]. These features are designed to prevent [data races] --- situations in which multiple threads access the same memory without proper synchronization, with at least one of the accesses modifying that memory.
 
 This chapter describes the traits, types, and concepts that Rust uses to express and enforce safe concurrency.
 
 [concurrent programs]: glossary.md#concurrent-program
+[data races]: glossary.md#data-race

src/glossary.md

@@ -64,6 +64,10 @@ may be made visible to other crates by marking them as public in the crate
 root, including through [paths] of public modules.
 [More][crate].
 
+### Data race
+
+A data race occurs when multiple threads attempt to access the same shared memory location concurrently.
+
 ### Dispatch
 
 Dispatch is the mechanism to determine which specific version of code is actually run when it involves polymorphism. Two major forms of dispatch are static dispatch and dynamic dispatch. Rust supports dynamic dispatch through the use of [trait objects][type.trait-object].