When does a crime become an act of 'terrorism'?

The attack on the UK parliament has been called 'terrorism'. Who decides whether a crime is a 'terror' act?

British police have said 52-year-old Khalid Masood was the man who carried out Wednesday's attack outside the parliament in London.

Police say Masood was born in the UK as Adrian Russell Ajao, and he used many aliases.

ISIL, also known as ISIS, has claimed that Masood was one of its so-called soldiers. But British police have not confirmed that he was affiliated with any group.

They have also not confirmed a motive and are trying to establish what he was exposed to.