For the regular frontend pages the content
block is defined in the app/design/frontend/base/default/layout/page.xml
:
<default translate="label" module="page">
<label>All Pages</label>
<block type="page/html" name="root" output="toHtml" template="page/3columns.phtml">
...
<block type="core/text_list" name="content" as="content" translate="label">
<label>Main Content Area</label>
</block>
...
for the adminhtml pages it is defined in the app/design/adminhtml/default/default/layout/main.xml
<default>
<block type="adminhtml/page" name="root" output="toHtml" template="page.phtml">
...
<block type="core/text_list" name="left" as="left"/>
<block type="core/text_list" name="content" as="content"/>
<block type="core/messages" name="messages" as="messages"/>
...
Note: For those who using enterprise theme package take a look at the app/design/frontend/enterprise/default/layout/page.xml
file (in the enterprise package)