Region is active = mark is active. But it is not necessarily highlighted.
The region is highlighted if active and transient-mark-mode is on.
The region is also highlighted temporarily if you set it using the mouse or using Shift + cursor keys, i.e., even if transient-mark-mode is off.
Except that if the region is empty you won't see any highlighting.
From my point of view, "active" applies only to transient-mark-mode. It has no sense when the mode is turned off. Any function whose behavior depends on whether the region is active only does so
when t-m-mode is enabled. As the Emacs manual (node Persistent Mark
) says: "When
Transient Mark mode is off, the mark is never deactivated"
But this is not how it is spoken of sometimes recently. The reason has to do with "temporary transient-mark-mode" (see the Elisp manual, node The Mark
).
(Keep in mind too that if there is not yet any mark in the buffer then there is also no region there.)