When explaining a concept, for example, recursion, is it valid to use a construction such as:
- Recursion is where a subroutine calls itself.
(from a comment): but in the abstract sense of "is defined as".
Yes, that is one of the meanings of "where". It is also one of the meanings of "when".
Dictionaries will usually provide this sense of meaning, the way that you are using it in your example. A usage dictionary, such as Merriam-Webster's Dictionary of English Usage (MWDEU), will often have an entry on this too.
For instance, in my copy of MWCDEU, page 776-7, is the entry "when, where". It starts out with:
when, where A substantial number of recent usage books and schoolbooks object to the use of is when or is where in framing definitions, which use is often described as childish or immature. . . .
and then goes on, presenting some old arguments by prescriptivists or older usage commentators. It also discusses some objections by some recent usage commentators to some kinds of specific usages.
It also presents some actual usage examples, such as:
A holiday is when you don't have to go to work -- Robert Carver, Times Literary Supp., 4 June 1993
A Proper Dipthong is where both the Vowels are sounded together; as oi in Voice -- A. Fisher, Grammar 1753
There's also this excerpt, MWCDEU page 777:
Evidence scattered throughout the OED shows that is when definitions have appeared in a wide variety of reference books. Most of these are specialized glossaries of such subjects as medicine, law, falconry, heraldry, music, and seamanship. But the construction was also used in standard encyclopedias and dictionaries:
- 3.) In elections, a plurality of votes is when one candidate has more votes than any other, but less than half -- Webster 1828
This formerly standard pattern of defining has dropped out of use in present-day lexicography, but it continues to flourish in less formal surroundings, as those shown above, and in speech:
- I've always said that power is when people think you have power -- Tip O'Neill with William Novak, Man of the House, 1987
There is more info in there, but you'll probably want to read the entry for yourself.