jQuery underscore notation
<p>Could anybody give me an explanation of the underscore in a jQuery plugin function? I have no idea of what "<em>_</em>" means. Example code below:</p>

<pre><code>$.fn.bgStretch=function(o){
    this.each(function(){
        var th=$(this),
            data=th.data('bgStretch'),
            _={
                align:'leftTop',
                altCSS:{},
                css:{
                    leftTop:{
                        left:0,
                        right:'auto',
                        top:0,
                        bottom:'auto'
                    },
                    rightTop:{
                        left:'auto',
                        right:0,
                        top:0,
                        bottom:'auto'
                    },
                    leftBottom:{
                        left:0,
                        right:'auto',
                        top:'auto',
                        bottom:0
                    },
                    rightBottom:{
                        left:'auto',
                        right:0,
                        top:'auto',
                        bottom:0
                    }
                },
                preFu:function(){
                    _.img
                        .load(function(){
                            _.checkWidthFu()
                            _.img
</code></pre>
