The term partial function application is defined in section 3.1.5 "Function calls" of the internal last call draft of XPath 2.1[1] as "A function call is a partial function application if one or more arguments is an ArgumentPlaceholder."  The term function call is defined in the same section as "a QName followed by a parenthesized list of zero or more arguments."

Section 3.2.2[2] then adds, "A dynamic function invocation is a partial function application if at least one of the arguments is an ArgumentPlaceholder...."

So the definition of partial function application is incomplete as it stands.  I would suggest changing the definition in 3.1.5 to read "A function call or dynamic function invocation is a partial function application if one or more arguments is an ArgumentPlaceHolder."

Of course, that would introduce a forward reference to "dynamic function invocation."

[1] http://www.w3.org/XML/Group/qtspecs/specifications/xquery-11/html/xpath-21.html#id-function-calls
[2] http://www.w3.org/XML/Group/qtspecs/specifications/xquery-11/html/xpath-21.html#id-dynamic-function-invocation