In one variable, a linear function can be written as follows ... linear equation because one of the variables will always be a constant (distance). A linear relationship can also be found ...

Basso (ON Semiconductor, Toulouse, France) has written a new book, Linear Circuit Transfer ... explaining what transfer functions are and how time constants characterize a circuit.

This is an interesting class of functions, but not nearly interesting enough for calculus purposes. While I do spend lots of time pushing the idea of linear approximations on my students ...