The word operandum, featured in this morning's post about highbrow spam, is as yet unknown to the Oxford English Dictionary. But nearly 50 years ago, it was the title of a paper by none other than B.F. Skinner — "Operandum", J. Exp. Anal. Behav. 5(2), 1962 — which reads in its entirety:
An editor recently objected to manipulandum as "not in the dictionary." (A form, manipulare, of which it could conceivably be the gerundive, is only late medieval Latin and therefore lacking in status.) Tolman, who seems to have introduced the term, defines manipulanda as "the characters of objects which support motor activity." In current usage, they are the objects themselves. Strictly speaking, the term does not describe a device such as a foot-pedal or a voice-key which is not operated by hand, but manipulate has long since been generalized beyond that restriction.
A better and simpler word can be derived from operari. A piece of apparatus operated by an organism could appropriately be called an operatus, but this is dangerously close to a pun and would certainly be confusing. The gerundive operandum ought to be considered, however, as an alternative to manipulandum. It is close to operant, and its similarity to manipulandum and operate makes its meaning obvious.
(The business about an "apparatus operated by an organism … appropriately called an operatus" seems to call out for the skills of W.S. Gilbert or Tom Lehrer. But I digress.)
Google Scholar records 3,190 publications and other documents using the word operandum. And Wallace Stevens used it in his 1954 poem "Looking across the fields and watching the birds fly":
1 Among the more irritating minor ideas
2 Of Mr. Homburg during his visits home
3 To Concord, at the edge of things, was this:
4 To think away the grass, the trees, the clouds,
5 Not to transform them into other things,
6 Is only what the sun does every day,
7 Until we say to ourselves that there may be
8 A pensive nature, a mechanical
9 And slightly detestable operandum, free
10 From man's ghost, larger and yet a little like,
11 Without his literature and without his gods …
I'm inclined to think that this is was an independent coinage, though for all I know Skinner and Stevens used to get together of an evening to swap neologisms. In any case, Oxford, you should put operandum on the list for a Word Induction Ceremony.
Manipulandum (with 5,890 Google Scholar hits) is already there, glossed as "Something which is manipulated; an object or feature designed for or capable of manipulation (esp. in a psychological test or experiment)."