Going from one major version to another (i.e. 2.5 to 3.x) is usually called an upgrade as Starting with Joomla 1.6, this is a one-click upgrade (more or less), thus keeping the same installation of Joomla.
Going from one major version to another (i.e. 2.5 to 3.x) is usually called an upgrade as Starting with Joomla 1.6, this is a one-click upgrade (more or less), thus keeping the same installation of Joomla.