What's "immobilized"?

[deuxhero]

It pops up in Monk's AC bonus and Uncanny Dodge, but Sepia Snake Sigil is the only instance of something explicitly making a target "immobilized" (a few monsters have notes that they attempt to immobilize opponents, but the methods range from "drop in a pit" to "render unconscious") and there does not appear to be a definition.

[spalding]

To go with that would immobilized mean simply getting stymied by a tanglefoot bag means a monk is effectively shut down... or a net? Or is it more than simply being unable to take a move action?

I agree with deuxhero that this is an area that could certainly use more clarification.