Takfir is the Arabic/Islamic word for declaring someone an unbeliever (kafir).
In a thread about takfir from a few years ago, Sen McGlinn wrote the following:
Takfir - which we don't have - is not the same as covenant-breaking...
--Sen McGlinn
https://old.reddit.com/r/bahai/comments/5xrtvd/bah%C3%A1%C3%AD_perspective_of_takfir/dekg3pm/
But Sen himself was on the receiving end of takfir by the UHJ. The UHJ declared him an unbeliever and removed him from the membership rolls. The UHJ's letter, which is quoted on Sen's blog, says the following:
As to the questions raised in your email letter of 10 December concerning the removal of Mr. Sen McGlinn from Bahá’í membership, the House of Justice wishes to assure you that such an action is not taken lightly...
Mr. McGlinn is, of course, entitled to his own views. But one cannot actively propagate ideas over a prolonged period that contradict explicit Bahá’í Teachings and still be considered a Bahá’í.
--Universal House of Justice
So, the UHJ says they removed Sen as a member of the Baha'i Faith, because someone who propagates ideas as he did cannot be "considered a Bahá’í". And then Sen (who is still a UHJ-believer) claims Baha'is don't practice takfir. I wonder what kind of mental gymnastics it took to reach this conclusion.