I don't think that it has to be an igneous intrusion, that is just the most common. One of my geologic dictionaries also says that it can be the rock surrounding a mineral or ore deposit. So, basically it is the rock that is intruded by pretty much anything, though you don't get intrusions of sedimentary rocks, and many metamorphic are due to the intrusion of igneous rocks.

I am always amazed at how many geo people there are out here... not quite like this in the midwest where I grew up!