Value $ Scope of ng-model Throw "Undefined" in the Controller

Seeing your code I realize that when your data is within <ion-content> fail, if you remove that label everything will work. Now, this should not happen, but I told you that maybe that's not the right way to order your project.

I recommend separating your code (which is by default as ordered by Ionic)

www
   templates
       login.html
       registro.html
   js
       app.js
       controllers.js

In the file controller.js you define all the controllers for each one of the views or templates that you have for example:

angular.module('starter.controllers', [])
.controller('LoginCtrl', function($scope, $http) {

    $scope.registrar = function() {
        $http.post("http://127.0.0.1:8080/php/Conexion.php", {
            correo: $scope.mail,
            pass: $scope.word,
        }).success(function(data) {
            console.log("exito");
            console.log($scope.mail);
        });
    }
})

.controller('RegistroCtrl', function($scope, $http) {

    $scope.nuevoUsuario = function() {
        $http.post("http://127.0.0.1:8080/php/RegistroUsuario.php", {
            tipo: $scope.tipo,
            cedula: $scope.cedula,
            nombres: $scope.nombre,
            apellidos: $scope.apellido,
            email: $scope.email,
            pass: $scope.contra,
            indicativo: $scope.indicativo,
            numero: $scope.numero,
        }).success(function(data) {
            console.log($scope.cedula);
            console.log($scope.nombre);
            console.log($scope.apellido);
            console.log($scope.email);
            console.log($scope.contra);
            console.log($scope.indicativo);
            console.log($scope.numero);
        });
    }

});

Then in the file app.js you inject the dependency of started.controllers which is what we defined previously.

angular.module('starter', ['ionic', 'ngCordova', 'starter.controllers'])
.config(function($stateProvider, $urlRouterProvider, $ionicConfigProvider) {
    $stateProvider

    .state('login', {
        url: '/login',
        cache: false,
        templateUrl: "templates/login_inicio.html",
        controller: "LoginCtrl"
    })

    .state('registro', {
        url: '/registro',
        cache: false,
        templateUrl: "templates/registro.html",
        controller: "RegistroCtrl"
    });
    $urlRouterProvider.otherwise('/login');
});

Defines the name of the view, the url it will take, the template it will occupy and which controller it will have. Finally your view would be something like this:

<ion-view view-title="Login" name="login-view" cache-view="false">
    <ion-content>
    <img class="indexImg" src="img/saludoIndex.jpg">
    <br><br>
          <div class="item item-input-inset">    
              <label class="item-input-wrapper">
                  <i class="icon ion-at placeholder-icon"></i>
                  <input type="text" placeholder="Dirección de email" ng-model="mail">
              </label>
          </div>

          <div class="item item-input-inset">    
              <label  class="item-input-wrapper">
                  <i class="icon ion-locked placeholder-icon"></i>
                  <input type="text" placeholder="Contraseña" ng-model="word">
              </label><br>
          </div>

          <div class="col text-center">
              <br><h4><a>¿Olvidaste tu contraseña?</a></h4><br><br><br><br><br>
              <a class="button button-stable button-large" href="templates/Register.html">
              <b>Crear una cuenta gratuita</b></a>
          </div>


  </ion-content>

    <ion-footer-bar class="bar-positive tabs">
        <a class="tab-item">
            <h4 style="color:white;" ng-click="registrar()">INICIAR SESIÓN</h4>
        </a>
    </ion-footer-bar>
</ion-view>

With this you define that for each view there is a controller, which handles the values of $scope . This is more modular your project, easy to read because everything is where it should be and there is nothing mixed.

I think the problem you have is with the scope scope. Try assigning the values to an object before passing it and then use the object instead of the scope.