Say, A (language expression) describes B (subject matter). But one claims that A determines the existence of B. I consider this an error/mistake/fallacy because I think B is prior to A. If I am right, is there a philosophical term for this kind of error/mistake/fallacy? Otherwise, for what am I wrong?
There is a related fallacy called post hoc ergo propter hoc, which is the incorrect statement that if A is after B, then A is caused by B.
The converse of post hoc ergo propter hoc is "If A is caused by B, then A is after B." The contrapositive of this is the equivalent statement "If A is not after B, then A is not caused by B." This is a true statement, and the concept it represents is termed the causal arrow of time. The fallacy you mentioned can be seen as a failure to satisfy this implication.
There is no cute Latin name because the fallacy was identified relatively recently, it is the same fallacy that underlies the ontological argument, the root of it is that existence is not a property. More precisely, no matter what properties are listed in a definition of an entity existence can neither be one of of them, nor can it be implied by one of them. In modern logic this is reflected by using a quantifier rather than a predicate to express existence.
The ontological argument roughly says that since God is defined as a perfect being he must exist because non-existence is a "deficiency" inconsistent with perfection. Some trace it to God telling Moses in the Bible (in creative translation) "I am that I am", in other words, I am defined by my existence. Kant objected however that existence adds nothing to the concept of an entity, including God, but merely indicates its occurrence in reality. So perfect being as an object of thought may very well be non-existent.
Even in mathematics, where at first glance definitions seem enough to "grant" existence, it is not so. Without going into detail about the nature of mathematical "existence", suffice it to say that one can define objects that do not exist even mathematically. A simple example is "the largest natural number", which tries to do with natural numbers what ontological argument tries to do with excellence. It does not exist because we can always add 1 to any natural number, and get a larger one. Similarly, we can have a chain of ever more excellent beings, but no perfect one. That's why implicit definitions in mathematics are usually followed by existence proofs before they are used, and those proofs invoke, directly or indirectly, independent existence postulates.
I would call that a debate between physicalism and idealism, though I do not have a cool Latin name for it.
If I can be very rough with my stereotypes, the idealist believes reality is an illusion created by our minds. In this sense, the mere fact that someone thought of [language expression] A causes the existence of [illusionary subject] B. A phyiscalist believes that consciousness is the illusion. In this sense, the existence of [subject] B causes the [illusionary idea] A.