glossary

src/doc/trpl/SUMMARY.md

@@ -36,3 +36,4 @@
     * [Macros](macros.md)
     * [Compiler Plugins](plugins.md)
 * [Conclusion](conclusion.md)
+* [Glossary](glossary.md)

src/doc/trpl/glossary.md

@@ -0,0 +1,17 @@
+% Glossary
+
+In this section we briefly explain terms that you may not be familiar with.
+
+This book avoids, as much as possible, overcomplicated terms. Nevertheless, the following were deemed important enough that omitting them would be a disservice to the reader.
+
+### Arity
+
+Arity refers to the number of arguments a function or operation takes.
+
+```rust
+let x = (2, 3);
+let y = (4, 6);
+let z = (8, 2, 6);
+```
+
+In the example above `x` and `y` have arity 2. `z` has arity 3.