Terrorism

Terrorism was the act of committing destructive acts with the purpose of spreading fear for political or religious reasons, participating individuals were known as terrorists.

On Earth
The World Trade Center was destroyed by terrorists on 11 September 2001. (AUDIO: The Longest Night, PROSE: Touched by an Angel)

The following year, several terrorist groups emerged in a period of political tension in 2002 brought about by the New Britannia Party. They included the United Front, Black Lightning and White Wolves. (AUDIO: The Fearmonger)

MI5 suspected that I2 was a front for the terrorist organisation Little Brothers. (PROSE: System Shock)

In the 21st century, a terrorist group known as Al-Qa'ida attempted to get nuclear weapons. (PROSE: The Nuclear Option)

As a cover for alien invasion
Terrorism was used, both by governments and civilians, to explain away alien invasions which occurred on Earth in the late 20th century and early 21st century.

Black Thursday was the cover story issued by the British government to hide the true nature of the Nestene's attempted invasion of Earth in the 1970s. It was used to explain that the Auton shop window dummies coming to life and the resulting deaths that followed this incident were because of terrorists, which many citizens theorised were of the Irish Republican Army. (PROSE: Who Killed Kennedy, TV: Spearhead from Space) A later Nestene invasion (TV: Rose) which occurred on 5 March 2005 would be covered up in the same way. (AUDIO: One Rule)

As Gwen Cooper told Jack Harkness, Rhys Williams believed that occurrences such as the spaceship over London on Christmas Day and the Battle of Canary Wharf were the result of "sort of terrorism.", suggesting that they had put psychotropic drugs in the water supplies, resulting in mass hallucinations. Jack replied "Yeah, well your boyfriend's stupid." (TV: Everything Changes)

When a fracturing of the Cardiff Space-Time Rift resulted in time slips occurring worldwide, initial news reports suggested terrorist involvement. When Rhys asked Gwen if she thought terrorists were responsible, Gwen replied "terrorists bomb things, this is different." (TV: End of Days)

Elsewhere in the universe
Angvian terrorists tried to disrupt a peace conference on Achilles 4 between the Angvians and Gholos. However, the real peace conference was happening on Dark Space 8. (AUDIO: Bang-Bang-a-Boom!)

V'rell was a Monan delegate to a major temporal summit on Gallifrey. Secretly he was a terrorist allied to the Free Time movement. (AUDIO: Square One)

A Tarenist cell known as the Church of Taren Capel claimed responsibility for the terrorist attacks that occurred during a three month period in Kaldor City. (AUDIO: Hidden Persuaders)

Other References
“Terrorist” has also been used as a derogatory term. Yasmin Khan was called a terrorist by a passerby on her way home from mosque. (TV: Rosa)

Behind the scenes
Very few acts of real-world terrorism have had an indirect impact on Doctor Who media. The initial broadcast of The Pyramid at the End of the World had a line of dialogue referring to the general act of terrorism removed in response to the which occurred five days prior. The, an attack motivated by Homophobia, prompted Class creator and writer Patrick Ness to reveal earlier than planned that the show would have an openly-gay lead, with Ness citing the early reveal being because it felt less important to keep it a secret in light of the attack.