Definition of "callable"

The list of server tasks is a compromise between only one task for all remotely callable subprograms and one task for each remotely callable subprogram.

1989, Angel Alvarez, Ada: The Design Choice

Historically, all viable frameworks have always provided a mechanism to implement callbacks. C# goes one step further and encapsulates callbacks into callable objects called delegates.

2010, Trey Nash, Accelerated C# 2010, page xxvi

The ctypes module can be used to create Python callables that wrap around arbitrary memory addresses.

2013, David Beazley, Brian K. Jones, Python Cookbook: Recipes for Mastering Python 3