What is the term for ability of a variable to hold a null value?

Question

In some programming languages, a variable may have a special value 'null' to denote an absence of any object. What term should be used to describe the property of whether or not a given variable can hold a 'null' value?

To further explain, if a variable can hold a 'null' value it is a 'nullable' variable. If it cannot, it is a 'non-nullable' variable, that is, assigning a 'null' value to such variable is a mistake.

What is the best term for such a property? Three possibilities are 'nullity', 'nullability' or 'nullness'. Which is preferable, or is there some other, even better word choice?

Here are some examples of each of three terms (with sources cited), which are contextually relevant to my use case:

• 'nullness': Eclipse IDE documentation

  Also a nullness default can be applicable at a method which is in conflict with an inherited null annotation.

• 'nullness': "Incremental Language Independent Static Data Flow Analysis" by Fabian Streitel PDF:

  We therefore define the nullness of a variable at a certain point in the source code as one of the following values...

• 'nullity': JML Reference Manual

  6.2.13 Nullity Modifiers Any declaration (other than that of a local variable) whose type is a reference type is implicitly declared non_null unless (explicitly or implicitly) declared nullable. <...>

• 'nullability': "Swift is open source and why you should care" by Paris Buttfield-Addison O'Reilly

  Because a variable's nullability can be determined at compile time, a huge number of null checks can be avoided.

• 'nullability': Wiktionary definition

  (computing) The state or property of being nullable

Answer 1

The ability for a variable to hold a null value can be described as "being nullable" or "having the property of nullability". The decision to use one term versus the other depends on the part of speech required by the specific sentence using the term.

The Wiktionary definition for nullability does have external references. To view the external references, click on the little down arrow where it says [quotations], and the references will appear as a drop-down. This external reference from the Wiktionary entry is a good example of how either nullable or nullability can be used, depending on the specific sentence in which you are using the term:

2006, Eric Gunnerson, A Programmer's Introduction to C# 2.0, page 271:

Because nullability is one of a number of attributes of any particular database field, and because dealing with changes in nullability is no different from dealing with name and data type changes, the idea of making a value type nullable in C# when it isn't nullable in the database just in case the underlying database schema changes makes little sense.

Answer 2

In programming terminology (your specific context—and stackoverflow.com might have been a better site for the question), such a name is already given—there is no need to reinvent it. At least according to Microsoft, what you're talking about is a value type that has a nullable structure.

Represents a value type that can be assigned null.

A type is said to be nullable if it can be assigned a value or can be assigned null, which means the type has no value whatsoever. By default, all reference types, such as String, are nullable, but all value types, such as Int32, are not.