$ 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>