(scientific computation) An eXTra option added to a routine without changing the calling sequence. For example, instead of adding an explicit input variable to instruct a routine to give eXTra diagnostic output, the programmer might just add a test for some otherwise meaningless feature of the existing inputs, such as a negative mass. The use of hidden flags can make a program very hard to debug and understand, but is all too common wherever programs are hacked in a hurry. [Jargon File] (1994-11-24)