In the context of a programming language with state (aka memory, or store in UK English), a value is storable if it can be assigned to a location in the store.
If higher-order functions can be assigned to a memory cell, like in ML-like languages, then we speak of higher-order state.
Strachey, Christopher. 1973. ‘The Varieties of Programming Language’. Technical Monograph PRG-10. Oxford: Oxford University Computing Laboratory. https://www.cs.ox.ac.uk/publications/publication3727-abstract.html.
@techreport{strachey_varieties_1973,
address = {Oxford},
type = {Technical {Monograph}},
title = {The {Varieties} of {Programming} {Language}},
url = {https://www.cs.ox.ac.uk/publications/publication3727-abstract.html},
number = {PRG-10},
institution = {Oxford University Computing Laboratory},
author = {Strachey, Christopher},
year = {1973}
}