The ability that Saiyajin have to transform into giant ape-like monsters.
Great Apes are giant ape-like creatures that Saiyans with tails have the power to transform into. The transformation into a Great Ape by a Saiyan is triggered by absorbing 17 million zeno units of Bruits Rays through the eyes. This can occur when a Saiyan looks directly at a planet or a full moon. If there isn't any of these in sight, some Saiyans are capable of generating a Power Ball which emits the same radiation as a full moon giving off enough radiation for a tailed Saiyan to trigger the Great Ape transformation. If a Saiyan's tail is cut off or the source of radiation triggering the transformation is destroyed, then a Great Ape returns to their humanoid Saiyan form.
These forms have the power to amplify a Saiyan's power tenfold and to fire destructive blasts of power from their mouths. The setback for this form is that a Great Ape loses all sense of reason and go berserk destroying everything around them. But it is possible for them to retain sense of who their allies and enemies are, as shown when Goku directs Gohan in Great Ape form to attack Vegeta during the final episodes of the Saiyan saga. Certain "elite" Saiyans are capable of retaining their sense of self in Great Ape form and even capable of speech in their transformed state.