"Ga" in geological terms stands for giga annum, which means billion years.

It is used to denote vast periods of time in geological history, particularly when discussing the age of rocks, fossils, and geological events.

For example:

* The Earth is approximately 4.5 Ga old.

* The extinction of the dinosaurs occurred about 66 Ma (mega annum) ago.

* The formation of the Himalayas happened around 50 Ma ago.