I recently read the Common Core standards for Math for grade 6, which is where the concept of variable appears to be introduced. **The standard in question** reads:

Write, read, and evaluate expressions in which letters stand for numbers.

I tweeted this out, and one person responded with this observation:

@davidwees in d/dx[f(x)]=mx+b …d, f, dx, f, x, m, and b all have completely different meanings, and only m and b “stand for numbers”

— John Chase (@mrchasemath) October 27, 2013

Clearly this definition of variable is limited, and possibly misleading.

How do you define variable with your students? How do you introduce the topic?