I was reading the novel Verity, and I came across the phrase "slightly ajar door". I didn't know what ajar meant and I looked it up to find it means (of a door) slightly open. I wonder if "slightly ajar" is a tautology and is grammatically incorrect? Since it is used by the author who must be much better than me at english to make such a mistake, I wonder if tautology is also a writing tool used by the author or is it simply a mistake?
Redundancy or a pleonasm can be a device used by an author. That doesn't make it a grammar mistake. In some contexts, redundancy is avoided, but in many contexts it is just a natural aspect of language, and occurs in every natural language (including your native language).
However, could there be different degrees of "being ajar"? I think there could be; the door could be ajar with a 20cm gap, or ajar with a 0.5 cm gap. So there is nothing actually redundant with saying that the door is "slightly ajar".
The phrase "slightly ajar" could be considered a bit redundant, but it's not necessarily a clear-cut tautology. "Ajar" itself means slightly open, so adding "slightly" before it might seem redundant, as "ajar" already implies a degree of openness. However, using "slightly" in this context could be seen as a way to emphasize just how minimally open something is. It's a matter of style and how the speaker or writer wants to convey the level of openness.
Tautology is the repetition of the same meaning in different words, often leading to redundancy. While "slightly ajar" could be seen as leaning toward redundancy, it's not as glaringly redundant as some other tautological phrases. It might be considered more of a stylistic choice than a clear-cut example of a tautology.
There are still useful gradations within ajar. To compare it to a more every day example someone may say something is ice cold - cold implies the temperature is below room temperature, but ice cold allows you to get a better idea of the range we're in.
Just as ajar lets us know the door is open a little, slightly ajar lets us know that, even within the range of ajar, the opening is particularly small.