While it was originally a theist term, it's now a euphemism for what theists do when they claim a lack of scientific knowledge proves God.
A perfect example is your comment about language, and saying that since science and linguistics can't explain everything about language, than that lack of knowledge proves God.
Theists do the same things in cosmology, biology, physics, and just about everywhere else. Where there's a lack of scientific knowledge (a gap), they insert God. Hence, To them God exists in the gaps in knowledge.
The problem for theists in using this argument is that science closes gaps all the time. Also, a gap in knowledge does not prove the supernatural, only that science has not yet come up with a satisfactory, natural explanation.
But you forget—atheistic scientism was the johnny come lately.
God and our knowledge of God precedes the infantile attempts of atheists to wipe him out of their mind.
It’s the mind of the 18 month old which needs concrete reassurance to avoid the phenomenon of “out of sight out of mind.” Atheists are not happy because when they look up into the sky the don’t see an old bearded man giving them the thumb’s up. (Actually their real problem is they know God is there but they don’t want to follow God’s rules.)
The only explanation for anything and everything is God. It’s atheism, not theism, that claims science replaces God.
This is why atheism collapses in the face of questions like what is the metaphysical explanation for language or information.