As the investigation of the Keystone Cops terrorist attempts in London and Glasgow proceeds, the plotters are not likely to turn out to be Al Qaeda members.  However, they are likely to be Takfiri Salafists–muslims sympathetic to the vision of Osama Bin Laden and his number two, Ayman Zawahiri.  Pat Lang, a scholar in his own right, first introduced me to this concept.   We need to understand why medical doctors would try to make car bombs (plus we should also now appreciate that just because you are a brain surgeon you don’t necessarily know the first thing about explosives.)  To understand this term I would direct you first to an excellent overview written by Trevor Stanley.  Stanley defines four critical terms–Kufr – Kaffir – Takfir – Takfiri.  The root word, k-f-r means:

Kufr
Unbelief (in Islam) – literally, ingratitude.

kafir

An infidel (non-Muslim).

takfir
Excommunication; declaring a person or group of people non-Muslim.

In mainstream Sunni Islam, it is considered wrong to engage in takfir. Sunni Islam has a general reluctance to spread fitna (sow dissension) or ‘backbite’. Furthermore, to declare takfir

is to pre-empt Allah’s judgement. The Muslim who considers another’s
actions to be wrong may say so, but will stop far short of declaring
that person an apostate from the faith. Similarly, there is a
reluctance to resist a leader who prays and does not restrict the
observance of the faith.

Even qualified mainstream religious scholars are reluctant to declare takfir except in particularly egregious cases.

Some radical groups have broken this taboo.

Takfiri
Those who excommunicate, or ‘declare kufr‘, mainstream Muslim individuals, societies and leaders.

Although nominally
Sunni, takfiris reject major aspects of mainstream Sunni religion. They
are also apt to reject components of society, culture and law in Muslim
countries, which they consider to have slipped back into a pre-Islamic
state of pagan ignorance (jahiliyya). Unsurprisingly, takfiris often support militancy against their regimes.

(Stanley’s full piece is at this link.)