Adding concurrent program definition to the glossary and referencing it in the concurrency chapter

Author: borsakv

src/concurrency.md

@@ -2,6 +2,8 @@ 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

src/glossary.md

@@ -50,6 +50,10 @@ Combinators are higher-order functions that apply only functions and
 earlier defined combinators to provide a result from its arguments.
 They can be used to manage control flow in a modular fashion.
 
+### Concurrent program
+
+A concurrent program is a program that can perform multiple tasks or processes at the same time, possibly overlapping in execution.
+
 ### Crate
 
 A crate is the unit of compilation and linking. There are different [types of