componentheading class is assigned to the DIV tag which displays the title of joomla components. Check out the following image. This class is defined almost in all of the joomla components.

Defined in:

Com_wrapper/wrapper.html.php
Com_weblinks/weblinks.html.php
Com_user/user.html.php
Com_search/search.html.php
Com_registeration/registeration.html.php
Com_poll/poll.html.php
Com_newsfeed/newsfeed.html.php
Com_login/login.html.php
Com_content/content.html.php
Com_contact/contact.html.php

Assigned to:

DIV Tag

example componentheading class