Terrorism is an action or threat designed to influence the government or intimidate the public. Its purpose is to advance a political, religious or ideological cause. The current UK definition of terrorism is given in the Terrorism Act 2006.

In the UK terrorism is defined as as a violent action that:

• Endangers a person’s life, other than that of the person committing the action
• Involves serious violence against a person
• Causes serious damage to property
• Creates a serious risk to the public’s health and safety
• Interferes with or seriously disrupts an electronic system

The UK Government’s Channel Duty Guidance, for example, defines extremism as the “vocal or active opposition to fundamental British values, including democracy, the rule of law, individual liberty and mutual respect and tolerance of different faiths and beliefs. We also include in our definition of extremism calls for deaths of members of our armed forces, whether in this country or overseas”.

There are a huge number of definitions of terrorism available as this is a highly contested term. One well-known definition is that contained in Title 22 of the United States Code, which states that terrorism is:
“…premeditated, politically-motivated violence perpetrated against non-combatant targets by sub-national groups or clandestine agents, usually intended to influence an audience.”

Violent jihadism is an ideology that aims to reorder government or society through the implementation by violence and oftentimes terrorism of Islamic or Sharia law.

Violent radicalisation (including online) is a process whereby individuals, through their online interactions and exposure to various types of internet content, come to view violence as a legitimate method of solving social and political conflicts. Some of those violently radicalised via the internet may go on to commit acts of terrorism.

Does terrorism differ from extremism?
The Counter Extremism Strategy 2015 says: “Extremism is the vocal or active opposition to our fundamental values, including democracy, the rule of law, individual liberty, and respect and tolerance for different faiths and beliefs. We also regard calls for the death of members of our armed forces as extremist.”

It’s important to remember that not all extremist groups, whether Islamist, far-right or other, will commit terrorist or violent acts. However, some groups pose particular threats, both online and offline.

Terrorist content can be described as material posted by or in support of organizations included on the Consolidated United Nations Security Council Sanctions List that depicts graphic violence, encourages violent action, endorses a terrorist organization or its acts, or encourages people to join such groups. The U.N. Sanctions List includes a list of groups that the U.N. Security Council considers to be terrorist organizations.

https://www.legislation.gov.uk/ukpga/2006/11/contents