Militant atheists is a derogatory term for atheists who are vocal about their anti-religious or secular views. Some define the term to apply to people that consider religion to be harmful or should be discouraged.  The term is also applied to leaders of oppressive regimes that used force to oppress religion, e.g. Stalin.
For instance, a newspaper compared 55 intellectuals and artists writing a letter to a newspaper saying 'Britain is not a “Christian country”' to the bigotry of religious extremists.
"The truth is that there is a new breed of militant atheists who are capable of being as unreasoning as the most bone-headed creationist. Their intolerance is a strange mirror reflection of the bigotry of religious extremists."
- — The Daily Mail, This new breed of militant atheists are as intolerant as any religious fundamentalists 
"The lady doth protest too much, methinks"! Some religious fundamentalists kill those who disagree with them; writing letter to a newspaper with a non-violent message is hardly equivalent.
Criticism of term
There is already an adequate word that can be used instead: secularist. While some writers have argued that they are distinct,  they are not (at least as far as "militant atheist" is actually meaningful).
The term "militant atheist" is misleading because it equates oppressive and violent dictators with peaceful writers and intellectuals who have expressed no indication that they would use force. "Militant" may also refer to aggression, but so-called militant atheists are not forcing anyone to listen to them or for their opponents to remain silent. Therefore "militant" is hardly an appropriate term and is a rhetorical guilt by association and ad hominem.
- "The police don't send undercover agents into sceptic societies and parliament doesn't pass emergency laws to combat atheist violence. Fanatics threaten European Muslims if they abandon their faith but no atheist will attack them if they keep it. No one thinks that atheists threaten the lives of their fellow citizens anywhere in the west. "
A. C. Grayling compared the term "militant atheist" to be as meaningful as a "militant non-stamp collector". He pointed out the reaction against atheists is caused not by the manner of atheists but by the content of what atheists are saying:
- "All we're doing is speaking very frankly and bluntly and they don't like it."
Secularism sometimes plays into the victim mentality of Christianity. Admittedly, religious groups are sometimes oppressed based on political, economic or ethnic motivations but these actions usually have little to do with atheism or secularism. Atheism is not a prescriptive moral system that can tell people how to act.