It sort of does though? Informed consent is very much a legal term, albeit one used almost exclusively within healthcare. Gorfias point that informed consent is not set in stone and not always mandatory is completely legit. In compulsory psychiatric care it can often be judged that informed consent can not be given and the treating psychiatrist has to decide what treatment has the best benefit versus how much damage it may cause (including psychological damage due to violations of personal autonomy and integrity). Similarly, as Gorfias pointed out, there are situations in emergency care when informed consent can not be properly obtained, either because the patient is unconscious or otherwise unable to provide it (being drugged out of their mind for example) or because the medical need is so great that you do not have time to obtain informed consent if you want to save the patients life. Then we get into the issues of whether informed consent can be given if the patient is under the influence, if they are in great pain or otherwise in such a medical situation that they want to get out of the suffering. What if we suspect they don't really understand the information we are giving?
On top of that, as Gorfias points out, there is the question of how much information informed consent actually entails. Does it mean explaining the intervention and common side effects and risks? A comprehensive discussion of potential outcomes and even the most low probability risks? If you say "I want you to take beta blockers, they lower your blood pressure" and the patients says "Great! Let's do it!" is that enough for informed consent? What if they suffer a massive blood pressure drop and dislocate their shoulder because you didn't have time to inform them about the side effects, are you liable because you didn't inform enough?
That we should always strive to gather informed consent is obvious and paramount and in most cases we can safely say that the patient has given informed consent before treatment or examination starts. But Gorfias point about the vagaries of informed consent is also correct. We should respect that as simple as it might seem to make sure the patient always agrees before we start our interventions, that will not always be the case because of the realities of healthcare. And that's why juries and judges eventually gets a say in how informed consent works.