At a physics conference I attended last week in Berlin, I came across yet another example of tricky distinctions to master. Several speakers confused the words monotonic or monotone with monotonous. While these words can be used interchangeably in some situations, only monotonic and monotone are correct in a mathematical context.
You can check the exact meaning using, for example, the Oxford online dictionary. For monotonous, you’ll find:
“never changing and therefore boring”
The fact that monotonic/monotone and monotonous translate to the same German word (monoton) does not exactly help. This is why we were taught in school that you cannot just take any possible translation from the dictionary but have to consider context. For example, entering monoton into the popular online dictionary leo.org produces monotonous, monotone, and monotonic (among other choices), with monotone and monotonic being clearly marked as mathematical expressions.