The name of the UI library that is going to be in use.
The css file for bootstrap should be manually included if you use bootstrap.
Examples
MaterialDesign
<select id="dropdown" width="200">
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
</select>
<script>
var dropdown = $('#dropdown').dropdown({
uiLibrary: 'materialdesign'
});
</script>
Bootstrap 3
<select id="dropdown" width="200">
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
</select>
<script>
$('#dropdown').dropdown({ uiLibrary: 'bootstrap' });
</script>
Bootstrap 4
<select id="dropdown" width="200">
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
</select>
<script>
$('#dropdown').dropdown({ uiLibrary: 'bootstrap4', width: 300 });
</script>