What is dumdum bullet?

In the 1890s, the British military developed a bullet to be used at Dum Dum arsenal in India’s North-West frontier and was named ‘dumdum’ bullet. It comprised a jacketed .303 bullet in which the jacket nose opened to expose its lead core to improve the effectiveness of the bullet by increasing its expansion upon impact. Afterwards, the term dumdum was taken to include any soft-nosed or hallow-pointed bullet. In 1899, the use of dumdum bullet was outlawed following the Hague Convention. But during First World War, the Germans accused the Belgian government of having used a dumdum bullet which the Belgians strongly denied.