In reference to web development and SEO, a title tag (also known as a meta title) is a HTML element that appears in the (header) section of a web pages HTML code. It does not appear on the page it self bar the tab.
The Title tag can be seen on:
The browser tab
As the headline in Search Engine Results
At the Title when sharing a page on Social Media (when opengraph tags do not exist)
The title tag is an important tag to optimise for SEO, it should be perfectly crafted to be the right length, contain the primary and secondary keywords, brand name and a suitable call to action.
The title tag looks like this in your source code:
My Page Title
Some older CMS systems will automatically output a Title tag based on the page name, however most allow you to customise the Title tag per page.
Whilst you may use a custom title tag, Search Engines like Google may re-write the Title tag for users, especially if they are overly optimised.