I am speaking of a barn or log cabin which has a window opening but no glass. Neither "window" nor "opening" seem quite right.
A fire blocked the cabin's only door. Fortunately, I squeezed through the _______ and escaped to safety.
Actually, a window ("wind-eye") was originally an unglazed opening to let in light and air ("wind"). In modern English it is still possible to use "window" for an opening without glass.
It's just called a window. If for some reason you really need to call attention to the fact that it doesn't have glass in it, you could call it an unglazed window.
Unglazed
- having no glass fitted
an unglazed circular skylight
https://www.collinsdictionary.com/us/dictionary/english/unglazed
As others have said you could just use the word window. If you want to emphasise that there is not glass pane in it, I would use frame or window frame.
the frame of a window that receives and holds the sashes or casements
Neither "window" nor "opening" seem quite right.
However, both "window" and "opening" when used together do.
A fire blocked the cabin's only door. Fortunately, I squeezed through the window opening and escaped to safety.
Construction sites and references I located use the window-modified term "opening" for the location where a gap for a window opening will be framed, cut, and installed.
Thereafter, the finished assembly continues to be referenced as a "window" which can be opened, closed, cleaned, broken, repaired, etc.
Given that the construction industry has a bias against incomplete structures, the term window refers more to the intended function of the feature rather than the actual construction and architectural treatment of it.
I would suggest glassless window but "window" does not, in itself, imply the presence of glass and is a correct description for :
an opening in a wall, door, roof or vehicle that allows the passage of light, sound, and/or air.
The origin is Middle English from the Norse vindauga : vindr ‘wind’ + auga ‘eye’. 'Window' replaced the Old English eagþyrl, which literally means 'eye-hole,' and 'eagduru' 'eye-door'. The word 'window' was first used in the 13th century.
There is also aperture
an opening, hole, or gap
Colloquially, 'aperture' would mean a small opening and glassless windows tend to be such, for the sake of shelter from wind and cold, so the word would be suitable.
Flat glass was made in Roman times but windows with glass only became common in the early 17th century. Prior to that, windows were made of flattened animal horn. Chaucer, writing towards the end of the 14th century, uses the word 'window' of a domestic home, when glassed windows would have been a rare sight, even in aristocratic dwellings.
The King James Bible (1611), translating a Hebrew word written in about 1000 BC, uses the word 'window' :
My beloved is like a roe or a young hart. Behold, he standeth behind our wall; he looketh forth at the windows, showing himself through the lattice. Song of Solomon 2:9.
Lattice, presumably, refers to a criss-cross of wooden slats.
I would use the phrase open window. Whether the window has glass or not by construction, an open window is such that a person, or wind, could pass through it without breaking any glass.
It seems like you don't want the construction to be part of the story, but you want the reader to know that there was no risk of being cut or sound of breaking glass. Therefore it seems like an avoidable problem if we let the reader assume the window does or does not have glass and concentrate the narrative on the fact that the character can use the opening to escape without trouble.
For an opening in the wall of a dwelling, especially a window (glazed or not), aperture may be used.
I believe the question has been answered. There is no single word that means "glassless window" or "unglazed window", most due to that fact that the word "window" predates the existence of common use of windows with glass.
However, in the deeper explanation of the question, was an another implied--but unasked--question, "How would you describe a glassless window, such as found in a primitive cabin?"
From a purely dramatic viewpoint, none of the suggestions--frame, hole in the wall, unglazed--work well. They disrupt the story flow. In this case, I believe that best solution is to establish the window is glassless before describing the action of jumping through it.
"A fire blocked the cabin's only door. Fortunately, I noticed there was no glass in the window and was able to squeezed through it and escaped to safety."
An opening to let in illumination, like a small hole/gap/window, can be called simply a "light" (see also "skylight" or "rooflight"). However that's quite a specialised architectural term IMO. I'd probably say "I squeezed through the empty window opening [...]".
In the Irish construction industry (I don't know about the broader world) it would be called an "ope" short for opening, although this would apply to doors too.