Santol is the name of the fruit of a tree that has the scientific name Sandoricum koetjape. It is native to Southeast Asia where it is called gratawn (กระท้อน) in Thai, kompem reach in Khmer, tong in Lao and donka in Sinhalese. The French refer to it as faux mangoustanier, while in English it's been called wild mangosteen or sandor. Previously, it had been given the scientific names Sandoricum indicum and Sandoricum nervosum.
The santol fruit has a skin that comprises a thin outer peel and a thicker inner rind. The pulp is soft and contains a milky juice. It may be sweet or sour depending on the ripeness. The seeds are brown and inedible.
Filipinos peel the fruit with a sharp knife and eat the flesh raw with some salt or even brown sugar. It may also be candied. Santol is a very popular fruit in the Philippines.
"sweetened" santol = candied santol
A popular idiomatic expression in Tagalog is the following rhetorical question:
Namumunga ba ng santol ang mangga?
Does a mango tree bear santol fruit?