The letter came in response to Severyn blaming the BBC for using the term in its article "Ukraine crisis: Yanukovych regrets bloodshed in Kiev." The relevant complaint was posted on Severyn's Facebook page.
Slamming the BBC's interview with Former Ukrainian President Viktor Yanukovych, the journalist alleged, in particular, that the events in eastern Ukraine should be described as "Russian aggression".
In his Facebook post, Severyn mentioned the BBC's response to his complaint about the impropriety of the term "civil war" in the definition of the conflict in the Donbass region.
The authors of the letter, for their part, expressed understanding of Severyn's anger about the term "civil war", but made it plain that the use of the term can be justified.
"The fighting pitches Ukrainian against Ukrainian over the future of their country and on a sufficient scale to justify our use of the term as and when appropriate," the letter said.
According to the Ukrainian newspaper Ukrainskaya Pravda, in the past few days similar letters have been sent to other Ukrainians who criticized the BBC for referring to the term "civil war". In particular, the letter was delivered to one of the employees of the British Embassy in Ukraine.
Kiev and the West have repeatedly claimed that Russia allegedly supports the self-proclaimed Donetsk and Lugansk People's Republics by sending troops and weapons to the Donbass region.
Moscow denies the allegations, insisting that the responsibility for the current events in eastern Ukraine lies with Ukrainian authorities.