Update Definition

Definition.md

@@ -55,3 +55,32 @@ The following evaluates to 5:
 x
 ```
 
+## defn
+
+macro: (defn _sym_ _lambda-list_ . _body_)
+
+defn defines a new function in the default global environment.
+
+defn is a shorter way to define functions. It is a macro defined in the standard library. The following code:
+
+```
+(defn my-add (a b)
+  (+ a b))
+```
+
+... is equivalent to a combination of `def` and `fn`, as seen below:
+
+```
+(def my-add (fn (a b) 
+                         (+ a b))) 
+```
+
+The body may be comprised of many forms. In this case, the entire body code is pasted directly into the `fn` form.
+As in `fn`, the entire body of the function will be executed when the declared function is called, and the
+value of the last expression executed is returned as the value of the function call. 
+
+## defmacro
+
+macro: (defmacro _sym_ _lambda-list_ . _body_)
+
+defmacro is similar to `defn`, with the exception that it defines a new macro in the default global environment.