$ is not a function jquery

There are quite lots of answer based on situation.

1) Try to replace '$' with "jQuery"

2) Check that code you are executed are always below the main jquery script.

<script src="http://code.jquery.com/jquery-1.11.3.min.js"></script>
<script type="text/javascript">
jQuery(document).ready(function(){

});
</script>
3) Pass $ into the function and add "jQuery" as a main function like below.

<script type="text/javascript">
jQuery(document).ready(function($){

});
</script>