terminology - What is modal about a modal dialog?

I'm reading a lot about modal dialogs.

I know(?) what modal means, having read Raskin's "The Humane Interface". I can understand what's modal about a toolbar in a drawing program (select mode, draw mode...), VI modes, caps lock... but in what way a modal dialog is modal?

I get that their intended use is for exceptional cases (errors, warnings), or when there's an unpostponable choice to make (ex: a sequence of actions as when we save an image and choose parameters and format). It seems that they force a single way of interaction (by negating all others), they're not changing the way input is interpreted. Is this to be intended as a mode?

Answer

I think your confusion here is based on homonyms (with similar meaning); you're taking "modal" to mean something related directly to "modality" which is a term in Human Computer interaciton:

In less formal terms, a modality is a path of communication between the human and the computer.

A modal window is called such because it has a distinct Mode or state of interaction, as the modal window capture all input rather than leaving the user in the "main" interface.

In user interface design, a mode is a distinct setting within a computer program or any physical machine interface, in which the same user input will produce perceived different results than it would in other settings.

Your modality or form of interaction is generally the same in and out of the modal window of a program, but in a modal window are in a distinct mode why which all interaction is captured.