jQuery is a fast, small, and feature-rich JavaScript library and we use it extensively across all our sites. A key factor when developing is that the library is correctly loaded, but also that it's only loaded once. To check if jQuery is already loaded into your platform (i.e. Wordpress has it by default) then you can use this Javascript snippet:-

If jQuery is present then you will see an alert with "jQuery is loaded!" and if it's not then 'jQuery is not loaded'.

How to add jQuery to your site

If jQuery is Not Loaded then simply add jQuery Script between your <head> tags:-</head>