Uncaught ReferenceError: firebase is not defined 오류

Firebase가 선언되지 않았다는 오류가 뜹니다... 해결방법 알려주시면 감사하겠습니다!

var firebaseConfig = {...};
  // Initialize Firebase
  firebase.initializeApp(firebaseConfig);

  $(document).ready(function (user){
      firebase.auth().onAuthStateChanged(function (user){
          if (user) {console.log('log',user);}
          else {console.log('err','not yet')}
      });
  });

  function login(){
    firebase.auth().createUserWithEmailAndPassword($("txtemail").val(), $("txtpasswd")).catch(function(error) {
        // Handle Errors here.
        var errorCode = error.code;
        var errorMessage = error.message;
        // ...
      });
      
  }