I think the definite article has to go in English; perhaps add, 'what is video calling' to make a more generic statement.
Ok, now I have added "a video call" as well as "video calling" and I've made it two words. Many thanks.
It is true, that this word, which was first used somewhere around about 1970, has increased in usage since 1970, with a relative spike in usage in the late 1980's. However the % of relevance is still only about 0.00000055%, and is currently falling in popularity.
We are considering your request, I do assure you that.
Why is it translated as “a” when the Greek uses the definite article in this sentence?
η βιντεοκλήση is used as a generic here -- the speaker is asking the listener to define or explain the concept of video calling or of videotelephony. A bit like η βιολογία είναι ενδιαφέρουσα for "biology is interesting".
But in English, we can't say "What is video call?" because "video call" isn't used as an abstract uncountable noun for the concept in the way that, say, "biology" or "videotelephony" is. So if you use "video call", you'd have to say "What is a video call?" even if you mean it generically.