By default, Joomla bases the HTML page title upon the menu link text (the menu item's Menu Title). However, you can also control the page title text manually. If you want the HTML page title to be different from the menu item title, you can set a specific HTML Page Title in the Menu Link details. Let's find out how this works:
Navigate to Menus | Menu Manager and select the menu containing the link you want to edit. In this example, let's select the About Joomla! menu.
Click on the Using Joomla link to edit it. In the Page Display Options panel, enter the desired Browser Page Title text. For testing purposes, you can enter something like Browser Page Title Set in Menu Item:
Click on Save & Close and then click on View Site to see the output on the frontend. Navigate to the menu item that you've changed to see the new title in the browser title bar (and in the current browser tab):