I think transcendental has different meanings depending on the context. There seem to be at least two different meanings:
Knowledge pertaining not to objects, but with the mode in which we perceive objects; as opposite of empirical, which pertains to objects of experience. Basically a "meta" discourse.
That which is independent of the conditions of human sensibility.
But in general, the term transcendental is connected to the conditions of human sensibility. The pure forms of human sensibility are transcendentally ideal, and the thing-in-itself is transcendentally real.
I don't know if I would describe transcendental as before or prior to experience. That would just be a priori, I think. The Transcendental Aesthetic is one part of the general question, "how are synthetic a priori judgements possible?", and it focuses on the conditions of sensibility, whereas the Transcendental Logic focuses on the conditions of thought. — darthbarracuda
I did some reading on the NKS commentary to the CPR for this points. NKS summaries 3 different definitions on transcendental of Kant in the CPR.
"1. Transcendental" is primarily employed by Kant as a name for a certain kind of knowledge. Transcendental is knowledge not of objects, but of the nature and condition of our a priori condition of them. In other words, a priori knowledge must not be asserted, simply because it is a priori, to be transcendental ; this title applies only to such knowledge as constitutes a theory or science of the a priori. Transcendental knowledge or transcendental philosophy must therefore be taken as coinciding ; and as thus coincident, they signify the science of possibility, nature and limits of a priori knowledge. The term similarly applies to the subdivisions of the Critique. The Aesthetic is transcendental in that it establishes the a priori character of the forms of sensibility ; the Analytic in that it determines the a priori principles of understanding, and the part which they play in the constitution of knowledge ....
But later in the critique Kant employs the term transcendental in the a second sense, namely, to denote a priori factors in knowledge. All representations which are a priori and yet are applicable to objects are transcendental. The term is then defined through its distinction from the empirical on the one hand, and from the transcendent on the other.
2. An intuition or conception is transcendental when it originates in pure reason, and yet at the same time goes to constitute an a priori knowledge of objects. The contrast between the transcendental and the transcendent, as similarly determined upon by Kant, is equally fundamental, but is quite of different character. That is transcendent which lies entirely beyond experience ; whereas the transcendental signifies those a priori elements which underlie experience as its necessary conditions. The transcendent is always unknowable. The transcendental is that which by conditioning experience renders all knowledge, whether a priori or empirical, possible. The direct opposite of the transcendent is the immanent, which as such includes both the transcendental and the empirical. ....
3. The third meaning of the term transcendental arises through its extension from the a priori intuitions and concepts to the processes and faculties to which they are supposed to be due. Thus Kant speaks of the transcendental syntheses of the apprehension, reproduction, and recognition, and of the transcendental faculties of imagination and undertaking. In this which render experience possible. And in as much as processes and faculties can hardly be entitled
a priori, Kant has in this third application of the term departed still further from this first definition of it." -
NKS Commentary to the CPR pp. 73-76 1922
I have based my interpretation of transcendental from the 2nd definition in NKS commentary to the CPR.
"The transcendental is that which by conditioning experience renders all knowledge, whether a priori or empirical, possible. The direct opposite of the transcendent is the immanent, which as such includes both the transcendental and the empirical."
That was a critical point, on which my understanding of transcendental in Transcendental Aesthetics was based.
The condition of experience must be prior / before to experience logically, otherwise it is not condition at all. If it is the same as experience then it would be experience itself, if after experience, then it would be the effect / result of experience.
Experience must always be the experience of something of someone. So it must have the subject of experience, and also the object of experience too. Experience in general before the real experience by the subject about something is a blank concept which has no meaning on its own, because there is no such an object which stands for experience in the real world. All experience is mental and has its subject and object to be meaningful.
Therefore the term a priori is also prior / before to experience to be meaningful, because on its own without matching real world experience of someone about something, it also is just a blank concept.
That was my understanding, but of course I imagine that it is subjective, and do expect possible criticisms.