Why doesn't allow "lang" as function name?

Question

I had getting error Uncaught TypeError: lang is not a function in calling function name as lang. Is lang something javascript system name ? It is working after I change function name something like lan !! Why was that ?

<script type="text/javascript">
    function lang(v){
          alert(v.value);
            //window.location = "?lang="+v.value;
    }
</script>  
<select onchange="lang(this);">
    <option>En</option>
    <option>Uk</option>
</select>