Full Idea
Second-order variables can range over properties, sets, or relations on the items in the domain-of-discourse, or over functions from the domain itself.
Gist of Idea
Second-order variables also range over properties, sets, relations or functions
Source
Stewart Shapiro (Higher-Order Logic [2001], 2.1)
Book Reference
'Blackwell Guide to Philosophical Logic', ed/tr. Goble,Lou [Blackwell 2001], p.33