Why is it called nillable?

Question

In an XML schema you can mark an element as nillable meaning it can take an explicit NULL value. See nillable and minOccurs XSD element attributes for a great e

Answer 1

What I'm wondering is why nil was chosen, rather than the more common (in computer science) null

This depends on which part of computer science you're coming from!

If you look at programs written in functional languages, you'll see nil every where, and very seldom null. And as it happens, XML and all it's siblings such as XSLT are closely related to functional languages.