Edit in JSFiddle

$("#search").click(function() { buscar(); });

function buscar(){
    var q = $("#searchterm").val();
    
    //DESACTIVO EL BOTON DE BUSCAR
    $("#search").attr("disabled", "disabled");    
    
    //REALIZO EL LLAMADO
    $.ajax({
	    type: "GET",
	    url: "http://en.wikipedia.org/w/api.php?action=parse&format=json&prop=text&section=0&page="+q+"&callback=?",
	    contentType: "application/json; charset=utf-8",
	    async: false,
	    dataType: "json",
	    success: function (data, textStatus, jqXHR) {	    
            var markup = data.parse.text["*"];
            $('#results').html(markup);		
	    },
	    error: function (errorMessage) {
	    }
	}).always(function() {
        //ACTIVO DE NUEVO EL BOTON BUSCAR
        $("#search").removeAttr("disabled");
    });
}

<input id="searchterm" type="text" />
<button id="search" >search</button>
<div id="results"></div>