If you're in a Browser-Only environment, use SridharR's solution.
If you're in a Node/CommonJS + Browser environment (e.g. electron, node-webkit, etc..); the reason for this error is that jQuery's export logic first checks for module, not window:
if (typeof module === "object" && typeof module.exports === "object") {
// CommonJS/Node
} else {
// window
}
Note that it exports itself via module.exports in this case; so jQuery and $ are not assigned to window.
So to resolve this, instead of <script src="path/to/jquery.js"></script>;
Simply assign it yourself by a require statement:
<script>
window.jQuery = window.$ = require('jquery');
</script>
NOTE: If your electron app does not need nodeIntegration, set it to false so you won't need this workaround.