Bootstrap CSS class .dropdown

<div class="dropdown">
  <button class="btn btn-secondary dropdown-toggle"
          type="button" id="dropdownMenu1" data-toggle="dropdown"
          aria-haspopup="true" aria-expanded="false">
    Dropdown
  </button>
  <div class="dropdown-menu" aria-labelledby="dropdownMenu1">
    <a class="dropdown-item" href="#">Action</a>
    <a class="dropdown-item" href="#">Another action</a>
  </div>
</div>