The Wikipedia article someone else posted already seems to disagree: https://en.wikipedia.org/wiki/Linear_function. This confuses me.
> In mathematics, the term linear function refers to two distinct but related notions (...) [proceeds to specify them using incomprehensible math words]
GNU Units documentation uses the second notion, common with linear algebra (and thus arguably more common in any computing) - the notion that calls y=ax a linear function, and y=ax+b an affine function.