Modal is a *(user interface) *(term), but it is applied from the field of logic, where a mode means a situation where only a finite set of possibilities for transitioning into the future are possible. That is, only certain paths are accessible. Modality in the logical sense has to do with the various phases of possibility, therefore. A modality and a mode are <em>not</em> the same concept.

We can say a _(user interface) is modeless if it always responds to the _(user)' input the same way. Thus, if it does not respond _(uniform)ly to a user's input the same way, it is modal. Keep in mind that this is a relative term, in that the behavior of a system in the user interface sense is subjective. That is, the user as subject has a model of the system which determines how she acts. So uniformity is relative; it represents conformance with the user's expectations based on how they perceive the system to be composed.

Modality causes problems two ways. First, _(user)s have an unstoppable tendency to habituate. It is impossible to prevent habituation, and it is difficult to break habits once they are formed. In addition, it is very difficult for a _(user) to stop herself from performing a habit. So often, she grows accustomed to one mode and treats a second mode as the first. This is nearly unavoidable, and means that even (or especially!) experts have a difficult time with modes. Second, people can only focus their attention on one place at a time. If, therefore, a _(user) is focused on the task at hand (as she should be), she cannot be focused on the modality of whatever interface she uses, and vice versa. The problem here is evident.
