Bootstrap class: .dropdown-header

<div class="dropdown open">
  <button class="btn btn-secondary dropdown-toggle"
          type="button" id="dropdownMenu3" data-toggle="dropdown"
          aria-haspopup="true" aria-expanded="false">
    Dropdown
  </button>
  <div class="dropdown-menu">
    <h6 class="dropdown-header">Dropdown header</h6>
    <a class="dropdown-item" href="#">Action</a>
    <a class="dropdown-item" href="#">Another action</a>
  </div>
</div>

Preview

Sass source

/* _dropdown.scss:152 */
.dropdown-header {
  display: block;
  padding: $dropdown-padding-y $dropdown-item-padding-x;
  margin-bottom: 0; // for use with heading elements
  font-size: $font-size-sm;
  color: $dropdown-header-color;
  white-space: nowrap; // as with > li > a
}

Uses variables

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/* _dropdown.scss:152 */
$dropdown-padding-y
$dropdown-item-padding-x
$font-size-sm
$dropdown-header-color

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